[go: up one dir, main page]
More Web Proxy on the site http://driver.im/

US6359933B1 - Frame synchronization in muticarrier transmission systems - Google Patents

Frame synchronization in muticarrier transmission systems Download PDF

Info

Publication number
US6359933B1
US6359933B1 US09/193,014 US19301498A US6359933B1 US 6359933 B1 US6359933 B1 US 6359933B1 US 19301498 A US19301498 A US 19301498A US 6359933 B1 US6359933 B1 US 6359933B1
Authority
US
United States
Prior art keywords
frame
synchronization
frame synchronization
receiver
symbols
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Expired - Lifetime
Application number
US09/193,014
Inventor
James T. Aslanis
Jacky S. Chow
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Texas Instruments Inc
Original Assignee
Texas Instruments Inc
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Family has litigation
First worldwide family litigation filed litigation Critical https://patents.darts-ip.com/?family=23052168&utm_source=google_patent&utm_medium=platform_link&utm_campaign=public_patent_search&patent=US6359933(B1) "Global patent litigation dataset” by Darts-ip is licensed under a Creative Commons Attribution 4.0 International License.
US case filed in New Jersey District Court litigation https://portal.unifiedpatents.com/litigation/New%20Jersey%20District%20Court/case/3%3A03-cv-02854 Source: District Court Jurisdiction: New Jersey District Court "Unified Patents Litigation Data" by Unified Patents is licensed under a Creative Commons Attribution 4.0 International License.
Application filed by Texas Instruments Inc filed Critical Texas Instruments Inc
Priority to US09/193,014 priority Critical patent/US6359933B1/en
Priority to US10/074,942 priority patent/US6912261B2/en
Application granted granted Critical
Publication of US6359933B1 publication Critical patent/US6359933B1/en
Priority to US10/757,195 priority patent/US20040199554A1/en
Anticipated expiration legal-status Critical
Expired - Lifetime legal-status Critical Current

Links

Images

Classifications

    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04LTRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00Modulated-carrier systems
    • H04L27/26Systems using multi-frequency codes
    • H04L27/2601Multicarrier modulation systems
    • H04L27/2647Arrangements specific to the receiver only
    • H04L27/2655Synchronisation arrangements
    • H04L27/2662Symbol synchronisation
    • H04L27/2665Fine synchronisation, e.g. by positioning the FFT window
    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04LTRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L5/00Arrangements affording multiple use of the transmission path
    • H04L5/02Channels characterised by the type of signal
    • H04L5/06Channels characterised by the type of signal the signals being represented by different frequencies
    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04LTRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00Modulated-carrier systems
    • H04L27/26Systems using multi-frequency codes
    • H04L27/2601Multicarrier modulation systems
    • H04L27/2647Arrangements specific to the receiver only
    • H04L27/2655Synchronisation arrangements
    • H04L27/2662Symbol synchronisation

Definitions

  • This invention relates to transmission systems using multicarrier modulation, and is particularly concerned with frame synchronization in such systems, referred to below for brevity simply as multicarrier systems.
  • multicarrier modulation The principles of multicarrier modulation are described for example in “Multicarrier Modulation For Data Transmission: An Idea Whose Time Has Come” by John A. C. Bingham, IEEE Communications Magazine, Vol. 28, No. 5, pages 5-14, May 1990.
  • FDM frequency division multiplexed sub-carriers spaced within a usable frequency band of a transmission channel, forming a set of sub-carriers, are modulated at a block or symbol transmission rate of the system.
  • the bits of input data for transmission within each block or symbol period are allocated to the sub-carriers in a manner which is dependent upon the signal-to-noise ratios (SNRs) of the sub-carriers, typically so that the bit error rates of the sub-carriers, as monitored at the receiver, are substantially equal.
  • SNRs signal-to-noise ratios
  • the different sub-carriers carry different numbers of bits in each symbol period. With an appropriate allocation of bits and transmit powers to the sub-carriers, such a system provides a desirable performance.
  • DMT discrete multitone
  • each frame corresponding to one block or symbol of the multicarrier system, for consistency with the same term as used in single carrier transmission systems. It should be appreciated that each frame, block, or symbol can comprise a substantial amount of information, for example about 1700 bits (providing a transmission rate of about 6:8 Mb/s with a symbol period of about 250 ⁇ s).
  • a single carrier transmission system for example a QAM (quadrature amplitude modulation) system, usually operates entirely in the time domain.
  • a relatively “random” frame synchronization sequence can be used to maintain frame synchronization, the sequence being inserted directly into the time-domain signal sample stream at the transmitter and being extracted and correlated with a stored copy of the sequence at the receiver.
  • a large correlation result indicates that frame synchronization has been maintained, and a small correlation result indicates a loss of frame synchronization, i.e. that there has been a slip by an unknown number of time-domain samples.
  • the receiver instigates a search procedure to resynchronize the receiver, i.e. to re-align the frame boundaries at the receiver to those at the transmitter.
  • This time domain frame synchronization provides a simple yes or no answer to the question of whether the receiver is frame synchronized.
  • the system may be required to correlate and search through a large number of possible frame alignments. This is a time-consuming, and hence undesirable, procedure.
  • An object of this invention is to provide an improved method of providing frame synchronization in a transmission system using multicarrier modulation, and an improved transmission system which makes use of this method.
  • One aspect of this invention provides a method of maintaining frame synchronization in a multicarrier modulation transmission system in which a synchronizing frame containing a synchronizing pattern is periodically transmitted, comprising the steps of: storing complex amplitudes of the synchronizing frame; correlating the complex amplitudes of the synchronizing frame with stored information representing the synchronizing pattern, thereby to produce a correlation result; and determining whether the correlation result falls below a threshold value, indicating a loss of frame synchronization, and in this event: performing a plurality of correlations between the stored information and the stored complex amplitudes in each case multiplied by a respective complex value representing a respective complex derotation of the stored complex amplitudes, each complex derotation corresponding to a respective time shift of the synchronizing frame, thereby to produce a plurality of correlation results each corresponding to a respective time shift; determining from the plurality of correlation results a time shift for restoring frame synchronization; and adjusting a frame boundary in accordance with the determined time shift
  • the method preferably includes the steps of: using a tone having a predetermined frequency for frequency synchronization between a transmitter and a receiver of the system; at the transmitter, converting complex amplitudes in the frequency domain into time domain values using an N-point Inverse Fast Fourier Transform, sampling time domain values at the transmitter at a sampling frequency which is j times the predetermined frequency, where j is an integral power of two; and at the receiver, converting time domain values into complex amplitudes in the frequency domain using an N-point Fast Fourier Transform; each of said complex derotations corresponding to a respective one of N/j time shifts within the duration of one frame.
  • the synchronizing frame is periodically transmitted once every Q frames, where Q is an integer greater than N/j, because it enables frame synchronization to be restored between two consecutive synchronizing frames.
  • each correlation result is produced by multiplying each complex amplitude by a corresponding complex amplitude from the stored information representing the synchronizing pattern, and summing the real parts of the complex products.
  • the method preferably includes the step of weighting the complex amplitudes being multiplied, the weighting for each complex amplitude being multiplied preferably being dependent upon a signal-to-noise ratio of a multicarrier channel associated with the respective complex amplitude.
  • a multicarrier modulation transmission system receiver comprising: a Fast Fourier Transform (FFT) unit for transforming time domain values into complex amplitudes in the frequency domain; a buffer for supplying received time domain values to the FFT unit in accordance with a frame boundary; a correlator for correlating complex amplitudes of a synchronizing frame of the system with a synchronizing pattern stored at the receiver to produce a correlation result; and a control unit responsive to the correlation result being below a threshold value to adjust the frame boundary by a time shift determined by performing a plurality of correlations between the stored synchronizing pattern and the complex amplitudes in each case multiplied by a respective complex value representing a respective complex derotation of the complex amplitudes corresponding to a respective time shift of the synchronizing frame, and selecting the best correlation result.
  • FFT Fast Fourier Transform
  • FIG. 1 illustrates parts of a transmission system using multicarrier modulation, in which frame synchronization is maintained and restored in accordance with an embodiment of this invention
  • FIG. 2 is a flow chart illustrating steps for maintaining and restoring frame synchronization in the transmission system of FIG. 1 .
  • a multicarrier system includes a DMT (discrete multitone) transmitter 10 and a DMT receiver 12 which are coupled via respective hybrid circuits 14 and 16 to a transmission path 18 , for example a two-wire telephone subscriber line, for communicating signals in what is referred to here as a downstream direction from the transmitter 10 to the receiver 12 .
  • An upstream transmitter (not shown, but which can be similar to the transmitter 10 ) is coupled to the hybrid circuit 16
  • an upstream receiver (not shown, but which can be similar to the receiver 12 ) is coupled to the hybrid circuit 14 , for communicating signals in the opposite, upstream, direction via the path 18 .
  • the system may be an ADSL (asymmetric digital subscriber line) system in which the transmitted bit rate is greater in the downstream direction than in the upstream direction.
  • the transmitter 10 is supplied via a path 20 with data to be transmitted, and comprises a coder 22 , a frame synchronizing sequence source 24 , an IFFT (Inverse FFT, or Inverse Fast Fourier Transform) unit 26 which for example implements a 512-point IFFT, a cyclic prefix adder 28 , and a unit 30 comprising a DAC (digital-to-analog converter) and filters whose output is coupled to the hybrid circuit 14 .
  • IFFT Inverse FFT, or Inverse Fast Fourier Transform
  • the receiver 12 comprises a unit 32 which is supplied with a received signal from the hybrid circuit 16 and comprises filters and an ADC (analog-to-digital converter), a time domain equalizer (TEQ) 34 , a buffer 36 , a unit 38 which implements a 512-point FFT, and a frequency domain equalizer (FEQ) and decoder unit 40 which reproduces the original data on an output path 42 .
  • the buffer 36 serves for serial-to-parallel conversion of signals for supply to the FFT unit 38 , the cyclic prefix not being supplied to the FFT unit 38 and thereby being removed.
  • Other parts of the receiver 12 illustrated in the drawing relate to frequency and frame synchronization and are described below.
  • the downstream signal on the path 20 is divided into frames and encoded by the coder 22 into frequency domain multicarrier symbols which are supplied to the IFFT, unit 26 .
  • Each frame of data is represented by a respective multicarrier symbol, which comprises a complex amplitude (i.e. two amplitudes, for real and imaginary signal components) for each of a number of sub-carriers or tones of the system.
  • the system may use 256 discrete tones or sub-carriers with frequencies of n ⁇ 4.3125 kHz, where n is a number of the tone or carrier from 1 to 256.
  • Each tone amplitude is allocated a variable number of bits of the signal, in accordance with a bit allocation scheme which for example can be as described in the related application by R. R.
  • the number of bits allocated to each tone amplitude in each multicarrier symbol period of for example about 250 ⁇ s can be zero (i.e. the tone is not being used for the signal) or can vary from a minimum number, for example 2 bits, to a maximum number, for example in a range from 10 to 16 bits.
  • a synchronizing frame containing a synchronizing sequence generated by the source 24 is periodically inserted into the data flow from the coder 22 to the IFFT unit 26 (a time domain version of the synchronizing sequence could alternatively be inserted between the units 26 and 28 ).
  • the synchronizing sequence is for example a pseudo-random sequence as described further below, the same sequence being provided for each synchronizing frame.
  • one specific tone in every frame is reserved as a pilot tone and carries no information, thereby providing a transmitted pilot tone which serves for frequency synchronization as described further below.
  • Each frequency domain multicarrier symbol is transformed into a time domain multicarrier symbol by the IFFT unit 26 .
  • the time domain multicarrier symbol thus comprises 512 real-valued time domain samples, which are supplied to the cyclic prefix adder 28 .
  • the cyclic prefix adder 28 supplies a resulting serial stream of for example 544 real-valued time domain samples to the DAC and filter unit 30 , which converts these samples into filtered analog signals which are transmitted via the hybrid circuit 14 to the transmission path 18 .
  • the 544 samples are constituted by the 512 samples supplied by the IFFT unit 26 , prefixed by a repetition of the last 32 of these samples added by the cyclic prefix adder 28 .
  • the signal received via the transmission path 18 is supplied by the hybrid circuit 16 to the filter and ADC unit 32 , to reproduce the 544 serial samples per multicarrier symbol which are supplied to the time domain equalizer (TEQ) 34 .
  • the TEQ 34 is a finite impulse response filter which serves to limit most of the impulse response to less than the duration of the cyclic prefix, so that subsequent removal of the cyclic prefix reduces interference between consecutive multicarrier symbols.
  • the equalized serial time domain sample stream is supplied to the buffer 36 , which produces at its parallel output the 512 time domain samples of each multicarrier symbol, the 32 bits of the cyclic prefix thereby being removed.
  • These 512 time domain samples are supplied to the 512-point FFT unit 38 and are transformed by this unit to a frequency domain multicarrier symbol, comprising 256 complex tone amplitudes, which is supplied to the frequency domain equalizer (FEQ) in the unit 40 .
  • FEQ frequency domain equalizer
  • the FEQ comprises a complex signal one-tap adaptive equalizer for each of the 256 tones.
  • the FEQ and decoder unit 40 can for example be of the form illustrated in FIG. 3 of the related application by R. R. Hunt et al. referred to above.
  • the unit 40 produces the resulting decoded received signal on the outgoing data path 42 .
  • the transmitter 10 and receiver 12 can also implement variable delay buffering and trellis coded modulation, as described for example in the related application by J. M. Cioffi et aL referred to above.
  • the functions of the hybrid circuits 14 and 16 most or all of the functions at each of the transmitter 10 and the receiver 12 can be implemented by one or more digital signal processors.
  • the DAC in the transmitter unit 30 is supplied via a line 44 with a clock signal at a desired sampling frequency for operation of the DAC.
  • the ADC in the receiver unit 32 is required to be synchronized to a related frequency (the two frequencies can be exactly the same, or one can be an integer multiple of the other, or they may be related in a manner to accommodate rate conversion through sample stuffing, interpolation, or decimation). It is assumed here for simplicity that the ADC in the unit 32 operates at the sampling frequency of the DAC in the unit 30 .
  • To provide the frequency synchronization as indicated above one of the 256 tones is used exclusively as a pilot tone. Thus the allocation of bits of the signal on the path 20 to this tone is zero for each multicarrier symbol.
  • the sampling frequency and the pilot tone frequency are selected so that the sampling frequency is an integral power of two times the pilot tone frequency.
  • the pilot tone can have a constant phase, or it can carry over successive multicarrier symbols a specific phase pattern or long pseudo-random sequence which is known to both the transmitter and the receiver.
  • the IFFT unit 26 is supplied with a complex amplitude for the pilot tone which represents the desired contents of the pilot tone. For simplicity and convenience, it is assumed here that the pilot tone has a constant phase, and accordingly the IFFT unit 26 is supplied with a constant complex amplitude, representing this constant phase, for the pilot tone.
  • the receiver 12 includes a voltage controlled crystal oscillator (VCXO) 46 which produces on a line 48 a sampling clock signal for the ADC in the unit 32 , synchronized to the 2.208 MHz sampling frequency of the transmitter 10 by a control loop which includes a phase comparator 50 and digital and analog control loop filters represented by a unit 52 .
  • the FEQ and decoder unit 36 supplies the phase information of the received pilot tone via a line 54 to the phase comparator 50 , and a stored reference phase is also supplied to the phase comparator 50 from a store 56 .
  • the phase comparator 50 produces at its output a digital phase error control signal which is filtered by digital and analog filters in the unit 52 to produce an analog control voltage; this is used to control the VCXO 46 to maintain frequency synchronization.
  • frame synchronization of the transmitted multicarrier symbols of data must also be maintained between the transmitter and receiver.
  • the same frame boundaries as are used for the multicarrier symbols at the input to the IFFT unit 26 in the transmitter 10 must be used for the FFT. unit 38 in the receiver 12 .
  • the frame boundaries are used by the buffer 36 to determine which sequences, each of 512 time domain samples, are supplied to the FFT unit 38 to be transformed into the respective frequency domain multicarrier symbols.
  • This number Q is selected to provide a balance between data carrying capacity of the system (for which a high value of Q is preferred) and frame resynchronization time (for which a low value of Q is preferred).
  • the synchronizing frame contains pseudo-random data which can be applied to the tones of the synchronizing frame multicarrier symbol in any of a variety of different ways. A description of one of these ways, given for example, follows.
  • each pair of bits provides a complex amplitude which is supplied to the IFFT unit 26 for the respective tone of the synchronizing frame.
  • the pilot tone is then overwritten with its own appropriate complex amplitude as described above.
  • Tones which may be allocated fewer than two bits per symbol in accordance with the bit allocations established for the system either can be discarded at the receiver or can have their amplitudes suppressed at the transmitter so that they are not transmitted, thereby conserving power at the transmitter and avoiding possible interference among tones due to imperfect equalization or filtering.
  • the bit allocation table which is provided at both the transmitter and the receiver can be used to provide an energy scaling vector for each tone, by which the complex amplitude provided for the tone in the synchronizing frame is multiplied before being supplied to the IFFT unit 26 .
  • a complementary scaling can be effected in the receiver 12 .
  • a stored pseudo-random sequence from the source 24 could simply be allocated to those tones for which the bit allocation table indicates that there is a sufficient SNR (signal-to-noise ratio), the sequence being truncated when all of the tones which are being used in the synchronizing frame have been allocated pseudo-random data.
  • the receiver stores the same bit allocation table as the transmitter, so that the desired correlation can be performed correctly.
  • the receiver 12 For frame synchronization, as shown in FIG. 1 the receiver 12 includes a synchronizing sequence source 58 which corresponds to and produces the same synchronizing sequence as the source 24 in the transmitter, and a correlator 60 and store 62 to which the contents of each received synchronizing frame (or what the receiver understands to be each received synchronizing frame, i.e. every Q-th frame or multicarrier symbol) are supplied from the output of the FEQ in the unit 40 .
  • the FEQ coefficients which are used for the synchronizing frame may be different from those used for the data frames.
  • the receiver further includes a weighting multiplier 64 via which the synchronizing sequence from the source 58 is supplied to the correlator 60 , a complex derotation multiplier 66 for supplying to the correlator 60 the received synchronizing frame contents multiplied by complex derotation values as described below, and a frame synchronization decision unit 68 .
  • the unit 68 is responsive to correlation results produced by the correlator 60 to determine the presence or absence of frame synchronization and, via a path 70 , to make corrective changes to the frame boundaries used by the buffer 36 when necessary as described below.
  • frame synchronization is established in a manner for example as discussed below. In subsequent normal operation, frame synchronization is maintained without any change of frame boundaries being required. As described below, in this normal operating situation the correlator 60 and decision unit 68 monitor the frame synchronization. In the event of a loss of frame synchronization (in the presence of frequency synchronization, indicating that the receiver 12 is receiving a signal via the path 18 ), frame synchronization must be restored. While this can be done (as in the prior art) by re-initializing the system, this is very undesirable because the initializing process is relatively slow, for example taking about 20 seconds, resulting in a substantial interruption in the operation of the system.
  • An actual loss of frame synchronization may arise for example as a result of a printed circuit card containing the transmitter 10 being pulled from an equipment rack (resulting in a loss of signal, and frequency synchronization, at the receiver 12 ) and then being reinserted (the signal and frequency synchronization thereby being restored at the receiver).
  • a loss of frame synchronization may also be indicated by the monitoring in the event that there is excessive noise which produces a poor correlation result, even though there is no actual loss of frame synchronization. In this case, no frame resynchronization is necessary or desired.
  • the invention permits a distinction to be made between these situations, and in the event of an actual loss of frame synchronization generally enables frame synchronization to be restored, and thereby maintained without any re-initialization of the system, within a very short period for example of less than about 100 ms.
  • each synchronizing frame i.e. each 69-th frame or multicarrier symbol
  • the received contents of each synchronizing frame are supplied from the output of the FEQ in the unit 40 and stored in the store 62 . It is observed that these contents are complex amplitudes in the frequency domain, representing the complex amplitudes of the tones of the synchronizing frame.
  • these synchronizing frame contents are also supplied, either directly or from the store 62 , to the correlator 60 where they are correlated with the synchronizing sequence supplied from the store 58 via the weighting multiplier 64 .
  • This correlation consists of a multiplication of each complex amplitude supplied from the output of the FEQ in the unit 40 by a corresponding complex amplitude of the synchronizing sequence from the store 58 , weighted by the multiplier 64 in accordance with a respective weighting factor as described below, and a summation of the real parts of the complex amplitude products to produce a single real correlation result at the output of the correlator 60 .
  • the weighting coefficients used by the weighting multiplier 64 comprise either a binary 1 or 0 for each tone or complex amplitude, indicating that the tone respectively is or is not being used to contribute to the correlation.
  • the pilot tone always has a weighting coefficient of 0, because it is the reference for frequency synchronization, the control loop for which removes any phase errors from the pilot tone.
  • any other tones not currently being used i.e. with a bit allocation of zero
  • tones being used for transmission can have a weighting coefficient of 1. It can be seen that the weighting coefficients can be readily derived from the bit allocation table which is provided at the receiver 12 .
  • each coefficient used by the weighting multiplier 64 provides a weighting for the complex amplitude of the respective tone depending on the SNR (signal-to-noise ratio) of the respective tone.
  • This weighting can be based on the SNR for each tone as determined during initialization of the system (this being used in determining the bit allocations for the tones), or it can be based on the adaptive mean squared errors of the tones (which can be updated as frequently as every multicarrier symbol) which provide a measure of the current SNR for each tone, whereby the weighting coefficients are also adaptively updated.
  • the use of mean squared errors and their association with bit allocations and SNR are described in the related applications by R. R. Hunt et al. and P. S. Chow et al. referred to above.
  • the decision unit 68 determines whether the correlation result produced by the correlator 60 exceeds a threshold TL. In the normal situation of frame synchronization this will be the case, and no further action is taken.
  • FIG. 2 shows a return path 86 to the block 80 for the next synchronizing frame, and meanwhile data is supplied from the other frames or multicarrier symbols via the units 38 and 40 to the output path 42 in FIG. 1 .
  • the threshold TL is set at a relatively low value, so that the correlation result will generally exceed this even in the presence of substantial noise, so that false determinations of a loss of frame synchronization are substantially avoided.
  • a counter may be provided for requiring repeated failures of the correlation result to exceed the threshold TL in successive synchronizing frames before a loss of frame synchronization is determined.
  • the correlator 60 performs a correlation of the received synchronizing frame contents from the store 62 , multiplied in the complex derotation multiplier 66 by a respective set of complex derotations, with the synchronizing sequence from the source 58 weighted as described above. Consequently, the correlator 60 produces 64 correlation results, one in each of these 64 data frames. As shown by a block 90 in FIG. 2, the decision unit 68 determines a best one of these correlation results, and as shown by a block 92 in FIG. 2 determines whether this exceeds a resynchronization threshold TH.
  • the threshold TH is set at a higher level than the threshold TL, for example at about half the maximum possible correlation result for a frame synchronized state, so that false resynchronization results are substantially avoided.
  • a counter may be provided to require repeated similar results from the processes of the blocks 88 to 92 in successive superframes before a resynchronization is effected.
  • a block 94 in FIG. 2 is reached in which the unit 68 changes the frame boundary in a single step, by control of a pointer in the buffer 36 via the path 70 , as described further below.
  • resynchronization in response to a detected loss of frame synchronization can be effected in a single superframe, or in a few superframes if the counters mentioned above are also provided, whereby frame synchronization is substantially continuously maintained.
  • the superframe period is 17 ms. If both of the counters mentioned above are provided to have a required count of 2, the loss of frame synchronization is detected and resynchronization as described above is completed within four superframes, or 68 ms.
  • a block 98 in FIG. 2 is reached.
  • the value of the threshold TH can be reduced to increase the likelihood of an affirmative result in the block 92 , or a frame count can be changed by changing the pointer in the buffer 36 to examine a different frame for the synchronizing sequence, or the system may be re-initialized.
  • the processes of blocks 88 and 90 invariably result in an affirmative result in the block 92 ; thus the resynchronization process is highly effective.
  • W N e j2 ⁇ /N is the principal N-th root of unity, and the finite length sequence is then repeated periodically to form an N-periodic sequence, it can be shown that the time shift property is satisfied, i.e. f ⁇ ( k - m ) ⁇ ⁇ ⁇ DFT ; N ⁇ ⁇ W N - mn ⁇ F ⁇ ( n ) .
  • W N is a complex rotation, and thus the time shift of the periodic sequence f(k) results in complex rotations of the frequency domain samples of F(n), the amount of rotation depending on the frequency n and the time shift m.
  • the synchronizing sequence is not periodically repeated as an N-periodic sequence, but rather is preceded and followed by data from the path 20 as described above.
  • the data has a random nature relative to the synchronizing sequence, and the above characteristics can be applied to the synchronizing frame with reasonably accurate results.
  • this time shift can only be by an integer multiple of ⁇ 8 samples.
  • N 512 (512 point IFFT unit 26 and FFT unit 36 )
  • there are thus 512/8 64 possible time shifts per frame or multicarrier symbol.
  • Each of these possible time shifts is compensated by a respective one of the 64 sets of complex derotations used by the multiplier 66 , in accordance with the above equations.
  • each set of complex derotations by the multiplier 66 corresponds to a respective time shift of data samples in the buffer 36 by ⁇ 8, ⁇ 16, . . . ⁇ 256 samples. Because the rotation is recursive, only one set of complex derotations need be stored at the receiver 12 .
  • each of the 64 data frames referred to above is used for calculation of a correlation result for a respective one of the 64 possible time shifts m.
  • the complex amplitude for each tone n supplied from the store 62 is multiplied by the respective complex derotation W N ⁇ mn , and the resulting products are correlated in the correlator 60 with the weighted complex amplitudes of the synchronizing sequence supplied from the store 58 via the weighting multiplier 64 , the real parts of the correlation products being summed to produce the correlation result for the respective time shift m.
  • the correlation process is sufficiently accurate that, in the event that the loss of frame synchronization is due to one of the possible time shifts m being evaluated, the correlation result for that time shift exceeds the threshold TH whereas the correlation results for all other possible time shifts are much less than the threshold TH.
  • the decision unit 68 thereby reliably determines the time shift m which has produced the loss of frame synchronization, and via the path 70 as described above adjusts the pointer of the buffer 36 in a single step to correct this time shift, whereby frame synchronization is restored. This resynchronization is effected without any searching process for the synchronizing sequence.
  • this threshold can be lowered, or it can be concluded that a larger time shift has caused the loss of frame synchronization.
  • time shifts greater than one frame can be accommodated by changing the frame count to examine a different frame for the synchronizing sequence, the above steps then being repeated for the different frame count, and this search being continued for different ones of the 69 frames until the threshold TH is exceeded.
  • the system may be re-initialized. In either case a significant time delay is involved in restoring frame synchronization, but as stated above this event is unlikely in practice.
  • the initializing process includes a training method for the TEQ 34 in the receiver 12 , as described in an article by J. S. Chow et al. entitled “Equalizer Training Algorithms for Multicarrier Modulation Systems”, 1993 International Conference on Communications, pages 761-765, May 1993.
  • an equalized channel response b and an equalizer response (i.e. equalizer coefficients) w are obtained in the time domain by transformation by an IFFT.
  • the relative offset between the starting locations of b and w in the time domain determines the desirable delay for the received signal which in turn determines the multicarrier symbol or frame boundary at the receiver which is used to provide initial frame synchronization.
  • the correlation process described above can also be applied for initially establishing frame synchronization, each frame being used (during initialization) to carry the synchronizing sequence and a time shift being performed as described above in order to establish frame synchronization.
  • this process makes use of the FEQ coefficients (i.e. the correlation is of information derived from the output of the FEQ in the unit 40 , and thus presumes appropriate settings of the FEQ coefficients), it is in this case necessary first to compute an appropriate set of FEQ coefficients. This can be done from the channel response which is identified during the initializing process, the FEQ coefficients being determined so that the demodulated signal constellation is scaled and rotated into grids of fixed distance for ease of decoding.
  • frame synchronization can initially be established from the synchronizing sequence and the channel response determined during initialization by measuring the SNR for each tone for all of the 512 possible frame boundary alignments, and selecting as the frame boundary that alignment which results in the best SNR performance.
  • the sampling frequency is 8 times the pilot tone frequency, this need not be the case, but is preferred because this integer-power-of-two relationship considerably simplifies the digital signal processing which must be performed in the receiver 12 .
  • the invention has been described for only the downstream direction of transmission, it may equally be applied to the upstream direction of transmission, using either the same or (especially for an ADSL system having different rates of transmission and different IFFT and FFT sizes for the two transmission directions) different parameters.
  • the invention has been described in the specific context of DMT modulation, it may also be applied to transmission systems using other forms of multicarrier modulation.

Landscapes

  • Engineering & Computer Science (AREA)
  • Signal Processing (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Synchronisation In Digital Transmission Systems (AREA)
  • Mobile Radio Communication Systems (AREA)
  • Time-Division Multiplex Systems (AREA)

Abstract

A discrete multitone modulation transmission system is described in which frame synchronization is monitored at the receiver by correlating frequency domain complex amplitudes of a synchronizing frame with a stored synchronizing pattern. If the correlation result falls below threshold, indicating a loss of frame synchronization, a plurality of correlations are performed, in each case using the stored complex amplitudes of the synchronizing frame multiplied by a respective complex value representing a respective complex derotation corresponding to a respective possible time shift of the synchronizing frame. The best correlation result, if it exceeds another threshold, indicates a time shift for restoring frame synchronization, this being possible before the next synchronizing frame is received.

Description

RELATED APPLICATIONS
This is a Divisional application of prior U.S. application Ser. No. 08/770,702 now U.S. Pat. No. 5,901,180, filed on Dec. 19, 1996, which is a continuation application of prior U.S. application Ser. No. 08/275,409 filed on Jul. 15, 1994, (now U.S. Pat. No. 5,627,863), the disclosures of which are incorporated herein by reference.
Reference is directed to the United States patent applications identified below, the entire disclosure of each of which is hereby incorporated herein by reference:
U.S. Pat. No. 5,479,447 in the name of P. S. Chow et al. and entitled “Method And Apparatus For Adaptive, Variable Bandwidth, High-Speed Data Transmission Of A Multicarrier Signal Over Digital Subscriber Lines”, which describes details of a multicarrier system using DMT modulation.
U.S. patent application Ser. No. 08/107,200 filed Aug. 17, 1993 in the name of J. M. Cioffi et al. and entitled “Multicarrier Modulation Transmission System With Variable Delay”, which also describes details ofa multicarrier system using DMT modulation.
U.S. patent application Ser. No. 08/109,489 filed Aug. 20, 1993 in the name of R. R. Hunt et al. and entitled “Updating Of Bit Allocations In A Multicarrier Modulation Transmission System”, which describes a manner of maintaining bit allocations in a multicarrier system
This invention relates to transmission systems using multicarrier modulation, and is particularly concerned with frame synchronization in such systems, referred to below for brevity simply as multicarrier systems.
BACKGROUND OF THE INVENTION
The principles of multicarrier modulation are described for example in “Multicarrier Modulation For Data Transmission: An Idea Whose Time Has Come” by John A. C. Bingham, IEEE Communications Magazine, Vol. 28, No. 5, pages 5-14, May 1990. As is known, in a transmission system using multicarrier modulation, FDM (frequency division multiplexed) sub-carriers spaced within a usable frequency band of a transmission channel, forming a set of sub-carriers, are modulated at a block or symbol transmission rate of the system. The bits of input data for transmission within each block or symbol period are allocated to the sub-carriers in a manner which is dependent upon the signal-to-noise ratios (SNRs) of the sub-carriers, typically so that the bit error rates of the sub-carriers, as monitored at the receiver, are substantially equal. As a result, the different sub-carriers carry different numbers of bits in each symbol period. With an appropriate allocation of bits and transmit powers to the sub-carriers, such a system provides a desirable performance.
One particular form of multicarrier modulation, in which the modulation is effected using a discrete Fourier transform, is referred to as discrete multitone, or DMT, modulation. The related applications referred to above disclose details of multicarrier systems using DMT modulation.
As in any communication system, it is necessary to establish and maintain synchronization between the transmitter and receiver of a DMT or other multicarrier system. Frequency synchronization is conveniently provided in a DMT system by using one of the multiple tones as a pilot tone to control a phase locked loop at the receiver, as indicated in Standards Committee Contribution T1E1.4/93-022 by J. S. Chow et al. entitled “DMT Initialization: Parameters Needed For Specification In A Standard”, Mar. 8, 1993. This reference also outlines other initialization processes of a DMT system, including the allocation of bits to sub-carriers or tones of the system.
In addition to this frequency synchronization, synchronization of the transmitted blocks or symbols of data is required. This is referred to herein as frame synchronization, each frame corresponding to one block or symbol of the multicarrier system, for consistency with the same term as used in single carrier transmission systems. It should be appreciated that each frame, block, or symbol can comprise a substantial amount of information, for example about 1700 bits (providing a transmission rate of about 6:8 Mb/s with a symbol period of about 250 μs).
A single carrier transmission system, for example a QAM (quadrature amplitude modulation) system, usually operates entirely in the time domain. In such a system, a relatively “random” frame synchronization sequence can be used to maintain frame synchronization, the sequence being inserted directly into the time-domain signal sample stream at the transmitter and being extracted and correlated with a stored copy of the sequence at the receiver. A large correlation result indicates that frame synchronization has been maintained, and a small correlation result indicates a loss of frame synchronization, i.e. that there has been a slip by an unknown number of time-domain samples. In the latter case the receiver instigates a search procedure to resynchronize the receiver, i.e. to re-align the frame boundaries at the receiver to those at the transmitter.
This time domain frame synchronization provides a simple yes or no answer to the question of whether the receiver is frame synchronized. To resynchronize the receiver when frame synchronization is lost, the system may be required to correlate and search through a large number of possible frame alignments. This is a time-consuming, and hence undesirable, procedure.
An object of this invention is to provide an improved method of providing frame synchronization in a transmission system using multicarrier modulation, and an improved transmission system which makes use of this method.
SUMMARY OF THE INVENTION
One aspect of this invention provides a method of maintaining frame synchronization in a multicarrier modulation transmission system in which a synchronizing frame containing a synchronizing pattern is periodically transmitted, comprising the steps of: storing complex amplitudes of the synchronizing frame; correlating the complex amplitudes of the synchronizing frame with stored information representing the synchronizing pattern, thereby to produce a correlation result; and determining whether the correlation result falls below a threshold value, indicating a loss of frame synchronization, and in this event: performing a plurality of correlations between the stored information and the stored complex amplitudes in each case multiplied by a respective complex value representing a respective complex derotation of the stored complex amplitudes, each complex derotation corresponding to a respective time shift of the synchronizing frame, thereby to produce a plurality of correlation results each corresponding to a respective time shift; determining from the plurality of correlation results a time shift for restoring frame synchronization; and adjusting a frame boundary in accordance with the determined time shift to restore frame synchronization.
For a discrete multitone modulation transmission system, the method preferably includes the steps of: using a tone having a predetermined frequency for frequency synchronization between a transmitter and a receiver of the system; at the transmitter, converting complex amplitudes in the frequency domain into time domain values using an N-point Inverse Fast Fourier Transform, sampling time domain values at the transmitter at a sampling frequency which is j times the predetermined frequency, where j is an integral power of two; and at the receiver, converting time domain values into complex amplitudes in the frequency domain using an N-point Fast Fourier Transform; each of said complex derotations corresponding to a respective one of N/j time shifts within the duration of one frame. This is particularly advantageous if the synchronizing frame is periodically transmitted once every Q frames, where Q is an integer greater than N/j, because it enables frame synchronization to be restored between two consecutive synchronizing frames.
Preferably each correlation result is produced by multiplying each complex amplitude by a corresponding complex amplitude from the stored information representing the synchronizing pattern, and summing the real parts of the complex products. The method preferably includes the step of weighting the complex amplitudes being multiplied, the weighting for each complex amplitude being multiplied preferably being dependent upon a signal-to-noise ratio of a multicarrier channel associated with the respective complex amplitude.
Another aspect of this invention provides a multicarrier modulation transmission system receiver comprising: a Fast Fourier Transform (FFT) unit for transforming time domain values into complex amplitudes in the frequency domain; a buffer for supplying received time domain values to the FFT unit in accordance with a frame boundary; a correlator for correlating complex amplitudes of a synchronizing frame of the system with a synchronizing pattern stored at the receiver to produce a correlation result; and a control unit responsive to the correlation result being below a threshold value to adjust the frame boundary by a time shift determined by performing a plurality of correlations between the stored synchronizing pattern and the complex amplitudes in each case multiplied by a respective complex value representing a respective complex derotation of the complex amplitudes corresponding to a respective time shift of the synchronizing frame, and selecting the best correlation result.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be further understood from the following description with reference to the accompanying drawings, in which:
FIG. 1 illustrates parts of a transmission system using multicarrier modulation, in which frame synchronization is maintained and restored in accordance with an embodiment of this invention; and
FIG. 2 is a flow chart illustrating steps for maintaining and restoring frame synchronization in the transmission system of FIG. 1.
DETAILED DESCRIPTION
Referring to the drawing, a multicarrier system includes a DMT (discrete multitone) transmitter 10 and a DMT receiver 12 which are coupled via respective hybrid circuits 14 and 16 to a transmission path 18, for example a two-wire telephone subscriber line, for communicating signals in what is referred to here as a downstream direction from the transmitter 10 to the receiver 12. An upstream transmitter (not shown, but which can be similar to the transmitter 10) is coupled to the hybrid circuit 16, and an upstream receiver (not shown, but which can be similar to the receiver 12) is coupled to the hybrid circuit 14, for communicating signals in the opposite, upstream, direction via the path 18. For example, the system may be an ADSL (asymmetric digital subscriber line) system in which the transmitted bit rate is greater in the downstream direction than in the upstream direction.
The transmitter 10 is supplied via a path 20 with data to be transmitted, and comprises a coder 22, a frame synchronizing sequence source 24, an IFFT (Inverse FFT, or Inverse Fast Fourier Transform) unit 26 which for example implements a 512-point IFFT, a cyclic prefix adder 28, and a unit 30 comprising a DAC (digital-to-analog converter) and filters whose output is coupled to the hybrid circuit 14.
Conversely, the receiver 12 comprises a unit 32 which is supplied with a received signal from the hybrid circuit 16 and comprises filters and an ADC (analog-to-digital converter), a time domain equalizer (TEQ) 34, a buffer 36, a unit 38 which implements a 512-point FFT, and a frequency domain equalizer (FEQ) and decoder unit 40 which reproduces the original data on an output path 42. The buffer 36 serves for serial-to-parallel conversion of signals for supply to the FFT unit 38, the cyclic prefix not being supplied to the FFT unit 38 and thereby being removed. Other parts of the receiver 12 illustrated in the drawing relate to frequency and frame synchronization and are described below.
The downstream signal on the path 20 is divided into frames and encoded by the coder 22 into frequency domain multicarrier symbols which are supplied to the IFFT, unit 26. Each frame of data is represented by a respective multicarrier symbol, which comprises a complex amplitude (i.e. two amplitudes, for real and imaginary signal components) for each of a number of sub-carriers or tones of the system. For example, the system may use 256 discrete tones or sub-carriers with frequencies of n×4.3125 kHz, where n is a number of the tone or carrier from 1 to 256. Each tone amplitude is allocated a variable number of bits of the signal, in accordance with a bit allocation scheme which for example can be as described in the related application by R. R. Hunt et al. referred to above. The number of bits allocated to each tone amplitude in each multicarrier symbol period of for example about 250 μs can be zero (i.e. the tone is not being used for the signal) or can vary from a minimum number, for example 2 bits, to a maximum number, for example in a range from 10 to 16 bits.
For frame synchronization as further described below, a synchronizing frame containing a synchronizing sequence generated by the source 24 is periodically inserted into the data flow from the coder 22 to the IFFT unit 26 (a time domain version of the synchronizing sequence could alternatively be inserted between the units 26 and 28). For example, a synchronizing frame is provided as every Q=69-th frame or multicarrier symbol, so that each synchronizing frame is followed by 68 data fames. The synchronizing sequence is for example a pseudo-random sequence as described further below, the same sequence being provided for each synchronizing frame.
At the input to the IFFT unit 26, one specific tone in every frame is reserved as a pilot tone and carries no information, thereby providing a transmitted pilot tone which serves for frequency synchronization as described further below.
Each frequency domain multicarrier symbol is transformed into a time domain multicarrier symbol by the IFFT unit 26. The time domain multicarrier symbol thus comprises 512 real-valued time domain samples, which are supplied to the cyclic prefix adder 28. For each multicarrier symbol, the cyclic prefix adder 28 supplies a resulting serial stream of for example 544 real-valued time domain samples to the DAC and filter unit 30, which converts these samples into filtered analog signals which are transmitted via the hybrid circuit 14 to the transmission path 18. The 544 samples are constituted by the 512 samples supplied by the IFFT unit 26, prefixed by a repetition of the last 32 of these samples added by the cyclic prefix adder 28. The use and benefits of a cyclic prefix added in this manner are known for example from “A Discrete Multitone Transceiver System For HDSL Applications” by J. S. Chow et al., IEEE Journal on Selected Areas in Communications, Volume 9, No. 6, pages 895 to 908, August 1991.
In the receiver 12, the signal received via the transmission path 18 is supplied by the hybrid circuit 16 to the filter and ADC unit 32, to reproduce the 544 serial samples per multicarrier symbol which are supplied to the time domain equalizer (TEQ) 34. The TEQ 34 is a finite impulse response filter which serves to limit most of the impulse response to less than the duration of the cyclic prefix, so that subsequent removal of the cyclic prefix reduces interference between consecutive multicarrier symbols. The equalized serial time domain sample stream is supplied to the buffer 36, which produces at its parallel output the 512 time domain samples of each multicarrier symbol, the 32 bits of the cyclic prefix thereby being removed. These 512 time domain samples are supplied to the 512-point FFT unit 38 and are transformed by this unit to a frequency domain multicarrier symbol, comprising 256 complex tone amplitudes, which is supplied to the frequency domain equalizer (FEQ) in the unit 40.
The FEQ comprises a complex signal one-tap adaptive equalizer for each of the 256 tones. The FEQ and decoder unit 40 can for example be of the form illustrated in FIG. 3 of the related application by R. R. Hunt et al. referred to above. The unit 40 produces the resulting decoded received signal on the outgoing data path 42.
Although not described further here, the transmitter 10 and receiver 12 can also implement variable delay buffering and trellis coded modulation, as described for example in the related application by J. M. Cioffi et aL referred to above. Apart from the functions of the hybrid circuits 14 and 16, most or all of the functions at each of the transmitter 10 and the receiver 12 can be implemented by one or more digital signal processors.
The DAC in the transmitter unit 30 is supplied via a line 44 with a clock signal at a desired sampling frequency for operation of the DAC. The ADC in the receiver unit 32 is required to be synchronized to a related frequency (the two frequencies can be exactly the same, or one can be an integer multiple of the other, or they may be related in a manner to accommodate rate conversion through sample stuffing, interpolation, or decimation). It is assumed here for simplicity that the ADC in the unit 32 operates at the sampling frequency of the DAC in the unit 30. To provide the frequency synchronization, as indicated above one of the 256 tones is used exclusively as a pilot tone. Thus the allocation of bits of the signal on the path 20 to this tone is zero for each multicarrier symbol.
For convenience and simplicity, the sampling frequency and the pilot tone frequency are selected so that the sampling frequency is an integral power of two times the pilot tone frequency. For example, the 64-th tone (n=64) having a frequency of 64×4.3125=276 kHz is used as the pilot tone, and the sampling frequency is 8 times this pilot tone frequency, or 2.208 MHz. This relationship provides a particular advantage for the frame synchronization as described further below.
The pilot tone can have a constant phase, or it can carry over successive multicarrier symbols a specific phase pattern or long pseudo-random sequence which is known to both the transmitter and the receiver. The IFFT unit 26 is supplied with a complex amplitude for the pilot tone which represents the desired contents of the pilot tone. For simplicity and convenience, it is assumed here that the pilot tone has a constant phase, and accordingly the IFFT unit 26 is supplied with a constant complex amplitude, representing this constant phase, for the pilot tone.
The receiver 12 includes a voltage controlled crystal oscillator (VCXO) 46 which produces on a line 48 a sampling clock signal for the ADC in the unit 32, synchronized to the 2.208 MHz sampling frequency of the transmitter 10 by a control loop which includes a phase comparator 50 and digital and analog control loop filters represented by a unit 52. The FEQ and decoder unit 36 supplies the phase information of the received pilot tone via a line 54 to the phase comparator 50, and a stored reference phase is also supplied to the phase comparator 50 from a store 56. The phase comparator 50 produces at its output a digital phase error control signal which is filtered by digital and analog filters in the unit 52 to produce an analog control voltage; this is used to control the VCXO 46 to maintain frequency synchronization.
As explained in the background of the invention, frame synchronization of the transmitted multicarrier symbols of data must also be maintained between the transmitter and receiver. In other words, the same frame boundaries as are used for the multicarrier symbols at the input to the IFFT unit 26 in the transmitter 10 must be used for the FFT. unit 38 in the receiver 12. In the receiver 12 the frame boundaries are used by the buffer 36 to determine which sequences, each of 512 time domain samples, are supplied to the FFT unit 38 to be transformed into the respective frequency domain multicarrier symbols.
As described above, in the transmitter 10, every 68 data frames are supplemented by a synchronizing frame, thereby forming a superframe of Q=69 consecutive frames or multicarrier symbols. This number Q is selected to provide a balance between data carrying capacity of the system (for which a high value of Q is preferred) and frame resynchronization time (for which a low value of Q is preferred). The synchronizing frame contains pseudo-random data which can be applied to the tones of the synchronizing frame multicarrier symbol in any of a variety of different ways. A description of one of these ways, given for example, follows.
In the transmitter 10, a binary pseudo-random sequence of length 512 is produced by the source 24 in accordance with the equations: x [ p ] = 1 for p = 1 to 9 x [ p ] = x [ p - 4 ] x [ p - 9 ] for p = 10 to 512
Figure US06359933-20020319-M00001
where x[p] is the binary value of bit p of the sequence and ⊕ represents modulo-2 addition. The bits of this sequence are grouped into 256 pairs of bits, of which the first pair of bits is used for the d.c. and Nyquist sub-carriers (for which the allocated energy is zero so that this pair of bits is effectively ignored) and the remaining 255 pairs of bits are assigned to respective tones of the multicarrier symbol of the synchronizing frame in order of increasing frequency, the four possible combinations (0,0), (0,1), (1,0), and (1,1) of each pair of bits being mapped directly to 4-QAM points of the respective tone of the synchronizing frame. In other words, each pair of bits provides a complex amplitude which is supplied to the IFFT unit 26 for the respective tone of the synchronizing frame. The pilot tone is then overwritten with its own appropriate complex amplitude as described above.
Tones which may be allocated fewer than two bits per symbol in accordance with the bit allocations established for the system either can be discarded at the receiver or can have their amplitudes suppressed at the transmitter so that they are not transmitted, thereby conserving power at the transmitter and avoiding possible interference among tones due to imperfect equalization or filtering. In the latter respect, the bit allocation table which is provided at both the transmitter and the receiver can be used to provide an energy scaling vector for each tone, by which the complex amplitude provided for the tone in the synchronizing frame is multiplied before being supplied to the IFFT unit 26. A complementary scaling can be effected in the receiver 12.
As an alternative, a stored pseudo-random sequence from the source 24 could simply be allocated to those tones for which the bit allocation table indicates that there is a sufficient SNR (signal-to-noise ratio), the sequence being truncated when all of the tones which are being used in the synchronizing frame have been allocated pseudo-random data. Again, the receiver stores the same bit allocation table as the transmitter, so that the desired correlation can be performed correctly. As a further alternative, it is observed that not all of the available tones necessarily need be used in the synchronizing frame for the purposes of frame synchronization.
For frame synchronization, as shown in FIG. 1 the receiver 12 includes a synchronizing sequence source 58 which corresponds to and produces the same synchronizing sequence as the source 24 in the transmitter, and a correlator 60 and store 62 to which the contents of each received synchronizing frame (or what the receiver understands to be each received synchronizing frame, i.e. every Q-th frame or multicarrier symbol) are supplied from the output of the FEQ in the unit 40. The FEQ coefficients which are used for the synchronizing frame may be different from those used for the data frames. The receiver further includes a weighting multiplier 64 via which the synchronizing sequence from the source 58 is supplied to the correlator 60, a complex derotation multiplier 66 for supplying to the correlator 60 the received synchronizing frame contents multiplied by complex derotation values as described below, and a frame synchronization decision unit 68. The unit 68 is responsive to correlation results produced by the correlator 60 to determine the presence or absence of frame synchronization and, via a path 70, to make corrective changes to the frame boundaries used by the buffer 36 when necessary as described below.
When the transmission system including the transmitter 10 and the receiver 12 is initialized, frame synchronization is established in a manner for example as discussed below. In subsequent normal operation, frame synchronization is maintained without any change of frame boundaries being required. As described below, in this normal operating situation the correlator 60 and decision unit 68 monitor the frame synchronization. In the event of a loss of frame synchronization (in the presence of frequency synchronization, indicating that the receiver 12 is receiving a signal via the path 18), frame synchronization must be restored. While this can be done (as in the prior art) by re-initializing the system, this is very undesirable because the initializing process is relatively slow, for example taking about 20 seconds, resulting in a substantial interruption in the operation of the system.
An actual loss of frame synchronization may arise for example as a result of a printed circuit card containing the transmitter 10 being pulled from an equipment rack (resulting in a loss of signal, and frequency synchronization, at the receiver 12) and then being reinserted (the signal and frequency synchronization thereby being restored at the receiver). A loss of frame synchronization may also be indicated by the monitoring in the event that there is excessive noise which produces a poor correlation result, even though there is no actual loss of frame synchronization. In this case, no frame resynchronization is necessary or desired. The invention permits a distinction to be made between these situations, and in the event of an actual loss of frame synchronization generally enables frame synchronization to be restored, and thereby maintained without any re-initialization of the system, within a very short period for example of less than about 100 ms.
The operation of the components 58 to 68 is described further below with additional reference to the flow chart in FIG. 2.
In a frame synchronized state, as shown by a block 80 in FIG. 2 the received contents of each synchronizing frame, i.e. each 69-th frame or multicarrier symbol, are supplied from the output of the FEQ in the unit 40 and stored in the store 62. It is observed that these contents are complex amplitudes in the frequency domain, representing the complex amplitudes of the tones of the synchronizing frame. As shown by a block 82 in FIG. 2, these synchronizing frame contents are also supplied, either directly or from the store 62, to the correlator 60 where they are correlated with the synchronizing sequence supplied from the store 58 via the weighting multiplier 64. This correlation consists of a multiplication of each complex amplitude supplied from the output of the FEQ in the unit 40 by a corresponding complex amplitude of the synchronizing sequence from the store 58, weighted by the multiplier 64 in accordance with a respective weighting factor as described below, and a summation of the real parts of the complex amplitude products to produce a single real correlation result at the output of the correlator 60.
In the simplest case, the weighting coefficients used by the weighting multiplier 64 comprise either a binary 1 or 0 for each tone or complex amplitude, indicating that the tone respectively is or is not being used to contribute to the correlation. Thus for example the pilot tone always has a weighting coefficient of 0, because it is the reference for frequency synchronization, the control loop for which removes any phase errors from the pilot tone. Similarly, any other tones not currently being used (i.e. with a bit allocation of zero) can have a weighting coefficient of 0, and tones being used for transmission can have a weighting coefficient of 1. It can be seen that the weighting coefficients can be readily derived from the bit allocation table which is provided at the receiver 12.
More desirably, each coefficient used by the weighting multiplier 64 provides a weighting for the complex amplitude of the respective tone depending on the SNR (signal-to-noise ratio) of the respective tone. This weighting can be based on the SNR for each tone as determined during initialization of the system (this being used in determining the bit allocations for the tones), or it can be based on the adaptive mean squared errors of the tones (which can be updated as frequently as every multicarrier symbol) which provide a measure of the current SNR for each tone, whereby the weighting coefficients are also adaptively updated. The use of mean squared errors and their association with bit allocations and SNR are described in the related applications by R. R. Hunt et al. and P. S. Chow et al. referred to above.
As shown by a block 84 in FIG. 2, the decision unit 68 determines whether the correlation result produced by the correlator 60 exceeds a threshold TL. In the normal situation of frame synchronization this will be the case, and no further action is taken. FIG. 2 shows a return path 86 to the block 80 for the next synchronizing frame, and meanwhile data is supplied from the other frames or multicarrier symbols via the units 38 and 40 to the output path 42 in FIG. 1. The threshold TL is set at a relatively low value, so that the correlation result will generally exceed this even in the presence of substantial noise, so that false determinations of a loss of frame synchronization are substantially avoided. In addition, although not shown in FIG. 2, a counter may be provided for requiring repeated failures of the correlation result to exceed the threshold TL in successive synchronizing frames before a loss of frame synchronization is determined.
In the event that the correlation result does not exceed the threshold TL (in the requisite number of, e.g. 2, successive synchronizing frames); a block 88 in FIG. 2 is reached.
As represented by the block 88 and further described below, in each of the next 64 data frames, following the synchronizing frame for which a loss of frame synchronization has been determined, the correlator 60 performs a correlation of the received synchronizing frame contents from the store 62, multiplied in the complex derotation multiplier 66 by a respective set of complex derotations, with the synchronizing sequence from the source 58 weighted as described above. Consequently, the correlator 60 produces 64 correlation results, one in each of these 64 data frames. As shown by a block 90 in FIG. 2, the decision unit 68 determines a best one of these correlation results, and as shown by a block 92 in FIG. 2 determines whether this exceeds a resynchronization threshold TH. The threshold TH is set at a higher level than the threshold TL, for example at about half the maximum possible correlation result for a frame synchronized state, so that false resynchronization results are substantially avoided. Again in this case, but not shown in FIG. 2, a counter may be provided to require repeated similar results from the processes of the blocks 88 to 92 in successive superframes before a resynchronization is effected.
In response to the correlation result exceeding the threshold TH as determined in the block 92, a block 94 in FIG. 2 is reached in which the unit 68 changes the frame boundary in a single step, by control of a pointer in the buffer 36 via the path 70, as described further below. This change can be effected during the remaining 68−64=4 data frames, so that resynchronization is effected before, and can be confirmed with, the next synchronizing frame as shown in FIG. 2 by a path 96 from the block 94 to the block 80. Thus resynchronization in response to a detected loss of frame synchronization can be effected in a single superframe, or in a few superframes if the counters mentioned above are also provided, whereby frame synchronization is substantially continuously maintained. For example, with the sampling frequency of 2.208 MHz, 544 time domain samples in each Same, and 69 frames in each superframe as described above, the superframe period is 17 ms. If both of the counters mentioned above are provided to have a required count of 2, the loss of frame synchronization is detected and resynchronization as described above is completed within four superframes, or 68 ms.
If in the block 92 it is determined that no correlation result exceeds the threshold TH, then a block 98 in FIG. 2 is reached. In this block a variety of possible actions can be taken. For example, the value of the threshold TH can be reduced to increase the likelihood of an affirmative result in the block 92, or a frame count can be changed by changing the pointer in the buffer 36 to examine a different frame for the synchronizing sequence, or the system may be re-initialized. In practice, it has been found that the processes of blocks 88 and 90 invariably result in an affirmative result in the block 92; thus the resynchronization process is highly effective.
If the discrete Fourier transform of a finite length discrete sequence is defined as F ( n ) = k = 0 N - 1 f ( k ) W N - kn ,
Figure US06359933-20020319-M00002
where WN=ej2π/N is the principal N-th root of unity, and the finite length sequence is then repeated periodically to form an N-periodic sequence, it can be shown that the time shift property is satisfied, i.e. f ( k - m ) DFT ; N W N - mn F ( n ) .
Figure US06359933-20020319-M00003
WN is a complex rotation, and thus the time shift of the periodic sequence f(k) results in complex rotations of the frequency domain samples of F(n), the amount of rotation depending on the frequency n and the time shift m.
In the transmission system as described above, the synchronizing sequence is not periodically repeated as an N-periodic sequence, but rather is preceded and followed by data from the path 20 as described above. However, the data has a random nature relative to the synchronizing sequence, and the above characteristics can be applied to the synchronizing frame with reasonably accurate results.
A loss of frame synchronization in the system, while frequency synchronization is present as discussed above, corresponds to a time shift of the data samples supplied to the buffer 36 relative to the buffer pointer. In the system as described herein, in which the sampling frequency is 8 times the pilot tone used for frequency synchronization, this time shift can only be by an integer multiple of ±8 samples. With a symbol size of N=512 (512 point IFFT unit 26 and FFT unit 36), there are thus 512/8=64 possible time shifts per frame or multicarrier symbol. Each of these possible time shifts is compensated by a respective one of the 64 sets of complex derotations used by the multiplier 66, in accordance with the above equations. To accommodate time shifts in either direction, the 64 possible time shifts are used to represent time shifts by up to half a frame in each of the positive and negative directions. In other words, each set of complex derotations by the multiplier 66 corresponds to a respective time shift of data samples in the buffer 36 by ±8, ±16, . . . ±256 samples. Because the rotation is recursive, only one set of complex derotations need be stored at the receiver 12.
Thus for the block 88 in FIG. 2, each of the 64 data frames referred to above is used for calculation of a correlation result for a respective one of the 64 possible time shifts m. In the multiplier 66, the complex amplitude for each tone n supplied from the store 62 is multiplied by the respective complex derotation WN −mn, and the resulting products are correlated in the correlator 60 with the weighted complex amplitudes of the synchronizing sequence supplied from the store 58 via the weighting multiplier 64, the real parts of the correlation products being summed to produce the correlation result for the respective time shift m. The correlation process is sufficiently accurate that, in the event that the loss of frame synchronization is due to one of the possible time shifts m being evaluated, the correlation result for that time shift exceeds the threshold TH whereas the correlation results for all other possible time shifts are much less than the threshold TH. The decision unit 68 thereby reliably determines the time shift m which has produced the loss of frame synchronization, and via the path 70 as described above adjusts the pointer of the buffer 36 in a single step to correct this time shift, whereby frame synchronization is restored. This resynchronization is effected without any searching process for the synchronizing sequence.
In the event that no correlation result produced at the block 88 exceeds the threshold TH, then as indicated above this threshold can be lowered, or it can be concluded that a larger time shift has caused the loss of frame synchronization. In the latter case, time shifts greater than one frame can be accommodated by changing the frame count to examine a different frame for the synchronizing sequence, the above steps then being repeated for the different frame count, and this search being continued for different ones of the 69 frames until the threshold TH is exceeded. Alternatively, the system may be re-initialized. In either case a significant time delay is involved in restoring frame synchronization, but as stated above this event is unlikely in practice.
Frame synchronization must be established on initializing the system, as indicated above. The initializing process includes a training method for the TEQ 34 in the receiver 12, as described in an article by J. S. Chow et al. entitled “Equalizer Training Algorithms for Multicarrier Modulation Systems”, 1993 International Conference on Communications, pages 761-765, May 1993. At the end of the training of the TEQ 34, an equalized channel response b and an equalizer response (i.e. equalizer coefficients) w are obtained in the time domain by transformation by an IFFT. The relative offset between the starting locations of b and w in the time domain determines the desirable delay for the received signal which in turn determines the multicarrier symbol or frame boundary at the receiver which is used to provide initial frame synchronization.
Alternatively, the correlation process described above can also be applied for initially establishing frame synchronization, each frame being used (during initialization) to carry the synchronizing sequence and a time shift being performed as described above in order to establish frame synchronization. As this process makes use of the FEQ coefficients (i.e. the correlation is of information derived from the output of the FEQ in the unit 40, and thus presumes appropriate settings of the FEQ coefficients), it is in this case necessary first to compute an appropriate set of FEQ coefficients. This can be done from the channel response which is identified during the initializing process, the FEQ coefficients being determined so that the demodulated signal constellation is scaled and rotated into grids of fixed distance for ease of decoding.
As a further alternative, frame synchronization can initially be established from the synchronizing sequence and the channel response determined during initialization by measuring the SNR for each tone for all of the 512 possible frame boundary alignments, and selecting as the frame boundary that alignment which results in the best SNR performance.
The particular numbers, inter-relationships, and details given above obviously may all be varied to suit particular requirements. For example, although as described above the sampling frequency is 8 times the pilot tone frequency, this need not be the case, but is preferred because this integer-power-of-two relationship considerably simplifies the digital signal processing which must be performed in the receiver 12. Similarly, the size of Q=69 frames per superframe conveniently accommodates the correlation for the resulting 64 possible time shifts, up to ±256 samples, at a rate of one per frame, within the superframe, allowing additional time to implement the frame boundary shift before the next synchronizing frame as described above.
In addition, although the invention has been described for only the downstream direction of transmission, it may equally be applied to the upstream direction of transmission, using either the same or (especially for an ADSL system having different rates of transmission and different IFFT and FFT sizes for the two transmission directions) different parameters. Again, although the invention has been described in the specific context of DMT modulation, it may also be applied to transmission systems using other forms of multicarrier modulation.
Thus although a particular embodiment of the invention has been described in detail, it should be appreciated that these and numerous other modifications, variations, and adaptations may be made without departing from the scope of the invention as defined in the claims.

Claims (10)

What is claimed is:
1. A transmission system using multicarrier modulation, comprising:
a transmitter for transmitting symbols, said transmitter including a modulator for transforming complex amplitudes in the frequency domain to the time domain, a transmit symbol counter for counting the transmitted symbols, and a frame synchronization pattern source for providing a synchronization pattern to be transmitted as one of the transmitted symbols at a predetermined count of the transmit symbol counter; and
a receiver for receiving the transmitted symbols and obtaining data therefrom, said receiver including a demodulator for transforming the received time domain values into complex amplitudes in the frequency domain, a receive symbol counter for counting the received symbols, and a frame synchronizer for extracting at least one of the received symbols containing the synchronization pattern from said transmitter at a time dependent upon the count of the received symbol count.
2. A transmission system as recited in claim 1, wherein said transmitter transmits the transmitted symbols in a superfame, the superframe includes a plurality of data symbols and a synchronization symbol.
3. A transmission system as recited in claim 2, wherein the synchronization symbol is the last symbol of the superframe.
4. A transmission system as recited in claim 1, wherein said receiver utilizes the synchronization patterns extracted from the received symbols to monitor frame synchronization.
5. A transmission system as recited in claim 4, wherein said frame synchronizer monitors the frame synchronization by comparing the values of the receive symbols containing the synchronization pattern with corresponding values stored at said receiver for the synchronization pattern to produce a comparison result, and then comparing the comparison result with at least one threshold value to provide an indication of existence or loss of frame synchronization.
6. A transmission system as recited in claim 5, wherein said frame synchronizer monitors the frame synchronization by further determining an adjustment amount to restore frame synchronization when the frame synchronization has been lost, and then adjusting a frame boundary in accordance with the adjustment amount to restore frame synchronization.
7. A transmission system using multicarrier modulation, comprising:
a transmitter for transmitting symbols, said transmitter including a modulator for transforming complex amplitudes in the frequency domain to the time domain, a transmit symbol counter for counting the transmitted symbols, and a frame synchronization pattern source for providing a synchronization pattern to be transmitted as one of the transmitted symbols at a predetermined count of the transmit symbol counter; and
a receiver for receiving the transmitted symbols and obtaining data therefrom, said receiver including a demodulator for transforming the received time domain values into complex amplitudes in the frequency domain, a buffer for supplying the received time domain values to the demodulator in accordance with a frame boundary, a receive symbol counter for counting the received symbols, and a frame synchronizer for obtaining the at least one of the receive symbols containing the synchronization pattern from said transmitter at a time dependent upon the count of the received symbol count, for monitoring frame synchronization, and for updating the frame synchronization as needed.
8. A transmission system as recited in claim 7, wherein said frame synchronizer monitors the frame synchronization by comparing the values of the receive symbols containing the synchronization pattern with corresponding values stored at said receiver for the synchronization pattern to produce a comparison result, and then comparing the comparison result with at least one threshold value to provide an indication of existence or loss of frame synchronization.
9. A transmission system as recited in claim 8, wherein said frame synchronizer monitors the frame synchronization by further determining an adjustment amount to restore frame synchronization when the frame synchronization has been lost, and then adjusting a frame boundary in accordance with the adjustment amount to restore frame synchronization.
10. A receiver for a stream of frames, comprising:
a decoder for an input stream of frames with imbedded synchronizing frames;
a source for synchronizing frames;
a correlator for decoded ones of said received imbedded synchronizing frames and corresponding ones of said synchronizing frames from said source; and
a frame synchronizer with a low threshold, a high threshold, and a derotation multiplier;
wherein when output from said correlator is less than said low threshold (1) said frame synchronizer successively applies said derotation multiplier to said imbedded synchronizing frames prior to said correlator and (2) when the largest of outputs of said correlator exceeds said high threshold, said frame synchronizer adjusts frame boundaries corresponding to said largest of outputs of said correlator.
US09/193,014 1994-07-15 1998-11-16 Frame synchronization in muticarrier transmission systems Expired - Lifetime US6359933B1 (en)

Priority Applications (3)

Application Number Priority Date Filing Date Title
US09/193,014 US6359933B1 (en) 1994-07-15 1998-11-16 Frame synchronization in muticarrier transmission systems
US10/074,942 US6912261B2 (en) 1994-07-15 2002-02-11 Frame synchronization in multicarrier transmission systems
US10/757,195 US20040199554A1 (en) 1994-07-15 2004-01-14 Frame synchronization in multicarrier transmission systems

Applications Claiming Priority (3)

Application Number Priority Date Filing Date Title
US08/275,409 US5627863A (en) 1994-07-15 1994-07-15 Frame synchronization in multicarrier transmission systems
US08/770,702 US5901180A (en) 1994-07-15 1996-12-19 Frame synchronization in multicarrier transmission systems
US09/193,014 US6359933B1 (en) 1994-07-15 1998-11-16 Frame synchronization in muticarrier transmission systems

Related Parent Applications (1)

Application Number Title Priority Date Filing Date
US08/770,702 Division US5901180A (en) 1994-07-15 1996-12-19 Frame synchronization in multicarrier transmission systems

Related Child Applications (2)

Application Number Title Priority Date Filing Date
US10/074,942 Continuation-In-Part US6912261B2 (en) 1994-07-15 2002-02-11 Frame synchronization in multicarrier transmission systems
US10/074,942 Continuation US6912261B2 (en) 1994-07-15 2002-02-11 Frame synchronization in multicarrier transmission systems

Publications (1)

Publication Number Publication Date
US6359933B1 true US6359933B1 (en) 2002-03-19

Family

ID=23052168

Family Applications (5)

Application Number Title Priority Date Filing Date
US08/275,409 Expired - Lifetime US5627863A (en) 1994-07-15 1994-07-15 Frame synchronization in multicarrier transmission systems
US08/770,702 Expired - Lifetime US5901180A (en) 1994-07-15 1996-12-19 Frame synchronization in multicarrier transmission systems
US09/193,014 Expired - Lifetime US6359933B1 (en) 1994-07-15 1998-11-16 Frame synchronization in muticarrier transmission systems
US10/074,942 Expired - Fee Related US6912261B2 (en) 1994-07-15 2002-02-11 Frame synchronization in multicarrier transmission systems
US10/757,195 Abandoned US20040199554A1 (en) 1994-07-15 2004-01-14 Frame synchronization in multicarrier transmission systems

Family Applications Before (2)

Application Number Title Priority Date Filing Date
US08/275,409 Expired - Lifetime US5627863A (en) 1994-07-15 1994-07-15 Frame synchronization in multicarrier transmission systems
US08/770,702 Expired - Lifetime US5901180A (en) 1994-07-15 1996-12-19 Frame synchronization in multicarrier transmission systems

Family Applications After (2)

Application Number Title Priority Date Filing Date
US10/074,942 Expired - Fee Related US6912261B2 (en) 1994-07-15 2002-02-11 Frame synchronization in multicarrier transmission systems
US10/757,195 Abandoned US20040199554A1 (en) 1994-07-15 2004-01-14 Frame synchronization in multicarrier transmission systems

Country Status (12)

Country Link
US (5) US5627863A (en)
EP (1) EP0771496B1 (en)
JP (3) JP3303055B2 (en)
KR (1) KR100417815B1 (en)
CN (3) CN1163036C (en)
AU (1) AU696798B2 (en)
CA (3) CA2194972C (en)
DE (2) DE69513834T2 (en)
DK (1) DK0771496T3 (en)
ES (1) ES2144132T3 (en)
FI (1) FI115685B (en)
WO (1) WO1996002991A1 (en)

Cited By (16)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20020048333A1 (en) * 2000-05-25 2002-04-25 Nadeem Ahmed Joint detection in OFDM systems
US20020080824A1 (en) * 2000-07-28 2002-06-27 Wingrove Michael J. Notifying bit allocation changes in a multicarrier modulation communications system
US20020110208A1 (en) * 2001-02-09 2002-08-15 Alps Electric Co., Ltd. Frame synchronizing signal detecting method for reducing occurrence of error synchronization before link of frame synchronizing signal is established
US20020159553A1 (en) * 2001-04-17 2002-10-31 Comspace Corporation Method and apparatus for improving data frame synchronization in a low SNR environment
US20030103529A1 (en) * 2001-11-30 2003-06-05 Niklas Linkewitsch Deframer
US6785349B1 (en) * 1999-05-28 2004-08-31 3Com Corporation Correlation based method of determining frame boundaries of data frames that are periodically extended
WO2004093366A2 (en) * 2003-04-09 2004-10-28 Jabil Circuit, Inc. Method for initialization of per tone frequency domain equalizer (feq) through noise reduction for multi-tone based modems
US6813325B1 (en) * 1999-12-22 2004-11-02 Globespanvirata, Inc System and method to reduce transmit wander in a digital subscriber line
US6925112B1 (en) * 2002-07-26 2005-08-02 Jabil Circuit, Inc. Discrete multitone modem initialization system and method
US6993084B1 (en) * 1998-04-14 2006-01-31 Fraunhofer-Gesellschaft Zur Foerderung Der Angewandten Forschung E.V. Coarse frequency synchronisation in multicarrier systems
US20080056343A1 (en) * 2006-08-30 2008-03-06 Ravikiran Rajagopal Frame synchronization
US20100287402A1 (en) * 2009-05-11 2010-11-11 Electronics And Telecommunications Research Institute Timestamping apparatus and method
US20110317671A1 (en) * 2001-10-18 2011-12-29 Qualcomm Incorporated Multiple-access hybrid ofdm-cdma system
US9100261B2 (en) 2013-06-24 2015-08-04 Freescale Semiconductor, Inc. Frequency-domain amplitude normalization for symbol correlation in multi-carrier systems
US9106499B2 (en) 2013-06-24 2015-08-11 Freescale Semiconductor, Inc. Frequency-domain frame synchronization in multi-carrier systems
US9282525B2 (en) 2013-06-24 2016-03-08 Freescale Semiconductor, Inc. Frequency-domain symbol and frame synchronization in multi-carrier systems

Families Citing this family (168)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US6334219B1 (en) * 1994-09-26 2001-12-25 Adc Telecommunications Inc. Channel selection for a hybrid fiber coax network
DE4441566A1 (en) * 1994-11-23 1996-05-30 Bosch Gmbh Robert Method for digital frequency correction in multi-carrier transmission methods
US7280564B1 (en) 1995-02-06 2007-10-09 Adc Telecommunications, Inc. Synchronization techniques in multipoint-to-point communication using orthgonal frequency division multiplexing
USRE42236E1 (en) 1995-02-06 2011-03-22 Adc Telecommunications, Inc. Multiuse subcarriers in multipoint-to-point communication using orthogonal frequency division multiplexing
GB2302777B (en) * 1995-06-27 2000-02-23 Motorola Israel Ltd Method of recovering symbols of a digitally modulated radio signal
US6289064B1 (en) * 1996-03-07 2001-09-11 Matsushita Communication Industrial Co., Ltd. Synchronization equipment
US5822368A (en) * 1996-04-04 1998-10-13 Lucent Technologies Inc. Developing a channel impulse response by using distortion
US5748686A (en) * 1996-04-04 1998-05-05 Globespan Technologies, Inc. System and method producing improved frame synchronization in a digital communication system
US6002722A (en) * 1996-05-09 1999-12-14 Texas Instruments Incorporated Multimode digital modem
US5790514A (en) * 1996-08-22 1998-08-04 Tellabs Operations, Inc. Multi-point OFDM/DMT digital communications system including remote service unit with improved receiver architecture
US6771590B1 (en) * 1996-08-22 2004-08-03 Tellabs Operations, Inc. Communication system clock synchronization techniques
US6118758A (en) * 1996-08-22 2000-09-12 Tellabs Operations, Inc. Multi-point OFDM/DMT digital communications system including remote service unit with improved transmitter architecture
US5995483A (en) * 1996-08-22 1999-11-30 Tellabs Operations, Inc. Apparatus and method for upstream clock synchronization in a multi-point OFDM/DMT digital communication system
ATE233452T1 (en) * 1996-09-02 2003-03-15 St Microelectronics Nv IMPROVEMENTS IN, OR RELATING TO, MULTI CARRIER TRANSMISSION SYSTEMS
US6493395B1 (en) * 1996-09-02 2002-12-10 Stmicroelectronics N.V. Multi-carrier transmission systems
JP2001505373A (en) 1996-09-02 2001-04-17 テリア アクティエ ボラーグ Improvements in or related to multi-carrier transmission systems
US6363128B1 (en) * 1996-09-02 2002-03-26 Stmicroelectronics N.V. Multi-carrier transmission systems
US5995568A (en) * 1996-10-28 1999-11-30 Motorola, Inc. Method and apparatus for performing frame synchronization in an asymmetrical digital subscriber line (ADSL) system
US6496543B1 (en) * 1996-10-29 2002-12-17 Qualcomm Incorporated Method and apparatus for providing high speed data communications in a cellular environment
JP2988398B2 (en) * 1996-11-27 1999-12-13 日本電気株式会社 Unique word differential detection system and demodulator
KR100221336B1 (en) 1996-12-28 1999-09-15 전주범 Frame harmonic apparatus and method of multi-receiver system
KR100230271B1 (en) * 1997-01-10 1999-11-15 윤종용 The fine fft window position recovery apparatus for ofdm system
SE517930C2 (en) * 1997-01-31 2002-08-06 Ericsson Telefon Ab L M Method and device for initial synchronization in a communication system
US6370156B2 (en) 1997-01-31 2002-04-09 Alcatel Modulation/demodulation of a pilot carrier, means and method to perform the modulation/demodulation
US5896425A (en) * 1997-02-24 1999-04-20 At&T Wireless Services Inc Non-uniformly spaced tones for synchronization waveform
US6088385A (en) * 1997-06-30 2000-07-11 Integrated Telecom Express Flexible and scalable rate ADSL transceiver and system
US6073179A (en) * 1997-06-30 2000-06-06 Integrated Telecom Express Program for controlling DMT based modem using sub-channel selection to achieve scaleable data rate based on available signal processing resources
US6065060A (en) * 1997-06-30 2000-05-16 Integrated Telecom Express Modular multiplicative data rate modem and method of operation
US6128335A (en) * 1997-06-30 2000-10-03 Integrated Telecom Express Software rate adaptable modem with forward compatible and expandable functionality and method of operation
US6442195B1 (en) 1997-06-30 2002-08-27 Integrated Telecom Express, Inc. Multiple low speed sigma-delta analog front ends for full implementation of high-speed data link protocol
US6092122A (en) * 1997-06-30 2000-07-18 Integrated Telecom Express xDSL DMT modem using sub-channel selection to achieve scaleable data rate based on available signal processing resources
US6400759B1 (en) 1997-06-30 2002-06-04 Integrated Telecom Express, Inc. Device driver for rate adaptable modem with forward compatible and expandable functionality
US6314102B1 (en) * 1997-07-10 2001-11-06 Alcatel Telecommunications system for providing both narrowband and broadband services to subscribers
US6011807A (en) * 1997-07-18 2000-01-04 Innova Corporation Method and apparatus for transmitting data in a high speed, multiplexed data communication system
EP0895387A1 (en) * 1997-07-28 1999-02-03 Deutsche Thomson-Brandt Gmbh Detection of the transmission mode of a DVB signal
DE19738780A1 (en) * 1997-09-04 1999-03-11 Thomson Brandt Gmbh Method and circuit arrangement for correcting phase and / or frequency errors of digital multicarrier signals
US6389062B1 (en) * 1997-09-17 2002-05-14 Texas Instruments Incorporated Adaptive frequency domain equalizer circuits, systems, and methods for discrete multitone based digital subscriber line modem
EP0903897B1 (en) * 1997-09-22 2001-10-31 Alcatel Method and arrangement to determine a clock timing error in a multi-carrier transmission system
US6101216A (en) * 1997-10-03 2000-08-08 Rockwell International Corporation Splitterless digital subscriber line communication system
US6563856B1 (en) 1998-07-08 2003-05-13 Wireless Facilities, Inc. Frame synchronization and detection technique for a digital receiver
US20020159598A1 (en) * 1997-10-31 2002-10-31 Keygen Corporation System and method of dynamic key generation for digital communications
SE511055C2 (en) * 1997-11-03 1999-07-26 Telia Ab Improvements associated with repressing card hold echoes
US6134283A (en) * 1997-11-18 2000-10-17 Amati Communications Corporation Method and system for synchronizing time-division-duplexed transceivers
US6088414A (en) * 1997-12-18 2000-07-11 Alcatel Usa Sourcing, L.P. Method of frequency and phase locking in a plurality of temporal frames
DE19800718A1 (en) * 1998-01-12 1999-07-22 Abb Research Ltd Process for the transmission of digital data
US20040160906A1 (en) 2002-06-21 2004-08-19 Aware, Inc. Multicarrier transmission system with low power sleep mode and rapid-on capability
US6151295A (en) * 1998-02-26 2000-11-21 Wavesat Telecom Inc. OFDM receiving system
US6480475B1 (en) 1998-03-06 2002-11-12 Texas Instruments Incorporated Method and system for accomodating a wide range of user data rates in a multicarrier data transmission system
DE19810558A1 (en) * 1998-03-11 1999-09-16 Siemens Ag Integratable radio receiver circuit for frequency modulated digital signals
US6631175B2 (en) * 1998-04-03 2003-10-07 Tellabs Operations, Inc. Spectrally constrained impulse shortening filter for a discrete multi-tone receiver
US7440498B2 (en) * 2002-12-17 2008-10-21 Tellabs Operations, Inc. Time domain equalization for discrete multi-tone systems
ES2389626T3 (en) 1998-04-03 2012-10-29 Tellabs Operations, Inc. Shortening filter for impulse response, with additional spectral restrictions, for transmission of multiple carriers
FI106592B (en) * 1998-05-07 2001-02-28 Nokia Multimedia Network Termi Method and arrangement to achieve and maintain symbol synchronization especially in an OFDM system
JP3480313B2 (en) * 1998-05-26 2003-12-15 富士通株式会社 Digital subscriber line transmission method and xDSL device
US6507585B1 (en) 1998-05-27 2003-01-14 3Com Corporation Multi-carrier LAN adapter device using frequency domain equalizer
US6704317B1 (en) 1998-05-27 2004-03-09 3Com Corporation Multi-carrier LAN modem server
US6891887B1 (en) 1998-05-27 2005-05-10 3Com Corporation Multi-carrier LAN adapter device using interpolative equalizer
US6603811B1 (en) 1998-05-29 2003-08-05 3Com Corporation Low complexity frequency domain equalizer having fast re-lock
US6377683B1 (en) 1998-05-29 2002-04-23 3Com Corporation Low complexity frequency domain echo canceller for DMT transceivers
US6618452B1 (en) * 1998-06-08 2003-09-09 Telefonaktiebolaget Lm Ericsson (Publ) Burst carrier frequency synchronization and iterative frequency-domain frame synchronization for OFDM
JP3622510B2 (en) 1998-06-19 2005-02-23 富士通株式会社 Digital subscriber line transmission method, ADSL transceiver, channel analysis system method, and ADSL apparatus
US6219377B1 (en) * 1998-06-29 2001-04-17 Legerity, Inc. Method and apparatus for generating tones in a multi-tone modem
US6795424B1 (en) 1998-06-30 2004-09-21 Tellabs Operations, Inc. Method and apparatus for interference suppression in orthogonal frequency division multiplexed (OFDM) wireless communication systems
US6560273B1 (en) * 1998-10-07 2003-05-06 Ericsson Inc. Delay searcher and delay trackers interaction for new delays assignment to rake fingers
DE19857821A1 (en) * 1998-12-15 2000-06-29 Siemens Ag Method and communication arrangement for the transmission of information using a multi-carrier method
US6456654B1 (en) * 1998-12-22 2002-09-24 Nortel Networks Limited Frame alignment and time domain equalization for communications systems using multicarrier modulation
US6452991B1 (en) * 1998-12-30 2002-09-17 Ericsson Inc. Systems and methods for acquiring channel synchronization in time division multiple access communications systems using dual detection thresholds
US6654429B1 (en) 1998-12-31 2003-11-25 At&T Corp. Pilot-aided channel estimation for OFDM in wireless systems
US6111919A (en) * 1999-01-20 2000-08-29 Intellon Corporation Synchronization of OFDM signals
US6487252B1 (en) * 1999-01-29 2002-11-26 Motorola, Inc. Wireless communication system and method for synchronization
US6985548B1 (en) * 1999-02-03 2006-01-10 Conexant Systems, Inc. System and method for timing recovery in a discrete multi-tone system
US6539063B1 (en) 1999-02-18 2003-03-25 Ibiquity Digital Corporation System and method for recovering symbol timing offset and carrier frequency error in an OFDM digital audio broadcast system
US7496132B2 (en) * 1999-03-15 2009-02-24 Kg Electronics Inc. Pilot signals for synchronization and/or channel estimation
US7643540B2 (en) * 1999-03-15 2010-01-05 Lg Electronics Inc. Pilot signals for synchronization and/or channel estimation
US7012906B1 (en) * 1999-03-15 2006-03-14 Lg Information & Communications, Ltd. Pilot signals for synchronization and/or channel estimation
KR100294711B1 (en) * 1999-03-15 2001-07-12 서평원 Frame Synchronization Method using Optimal Pilot Symbol
US6721299B1 (en) * 1999-03-15 2004-04-13 Lg Information & Communications, Ltd. Pilot signals for synchronization and/or channel estimation
KR100308151B1 (en) * 1999-06-19 2001-11-14 서평원 method of confirming a frame synchronization, at that time correlation results being sampled is used
US6791960B1 (en) 1999-03-15 2004-09-14 Lg Information And Communications, Ltd. Pilot signals for synchronization and/or channel estimation
US6074086A (en) * 1999-04-26 2000-06-13 Intellon Corporation Synchronization of OFDM signals with improved windowing
US6269132B1 (en) 1999-04-26 2001-07-31 Intellon Corporation Windowing function for maintaining orthogonality of channels in the reception of OFDM symbols
DE69938906D1 (en) * 1999-05-21 2008-07-24 Fujitsu Ltd Digital subscriber line method, apparatus and system using synchronous processing
JP3650390B2 (en) * 1999-05-21 2005-05-18 富士通株式会社 Digital subscriber line transmission method and xDSL apparatus
US6608864B1 (en) 1999-05-26 2003-08-19 3Com Corporation Method and apparatus for fault recovery in a decision feedback equalizer
SE521513C2 (en) * 1999-06-04 2003-11-11 Ericsson Telefon Ab L M Method and apparatus in a multi-transmission system
US6320901B1 (en) 1999-06-15 2001-11-20 National Semiconductor Corporation Method for fast off-line training for discrete multitone transmissions
US6930995B1 (en) 1999-06-23 2005-08-16 Cingular Wireless Ii, Llc Apparatus and method for synchronization in a multiple-carrier communication system by observing a plurality of synchronization indicators
US6252902B1 (en) * 1999-09-13 2001-06-26 Virata Corporation xDSL modem having DMT symbol boundary detection
US6233276B1 (en) 1999-09-13 2001-05-15 Virata Corporation XDSL modem having time domain filter for ISI mitigation
DE19944495C2 (en) * 1999-09-17 2002-01-03 Bosch Gmbh Robert Method for receiving radio signals over a radio channel
US7072412B1 (en) 1999-11-09 2006-07-04 Maurice Bellanger Multicarrier digital transmission system using an OQAM transmultiplexer
US20020008525A1 (en) * 2000-02-29 2002-01-24 Ernest Seagraves TTR phase change detection and hyperframe alignment for DSL
US7088765B1 (en) * 2000-03-15 2006-08-08 Ndsu Research Foundation Vector calibration system
JP3732707B2 (en) * 2000-03-16 2006-01-11 富士通株式会社 Resynchronization control device and resynchronization method in subscriber side communication device
US6529868B1 (en) * 2000-03-28 2003-03-04 Tellabs Operations, Inc. Communication system noise cancellation power signal calculation techniques
JP3440919B2 (en) * 2000-04-07 2003-08-25 日本電気株式会社 Multipath detection circuit
DE10026325B4 (en) * 2000-05-26 2006-01-19 Robert Bosch Gmbh Method for synchronizing OFDM symbols in broadcast transmissions
US8363744B2 (en) 2001-06-10 2013-01-29 Aloft Media, Llc Method and system for robust, secure, and high-efficiency voice and packet transmission over ad-hoc, mesh, and MIMO communication networks
WO2001097478A2 (en) * 2000-06-13 2001-12-20 At & T Wireless Services, Inc. Frame synchronization in a multicarrier communication system
US6975585B1 (en) 2000-07-27 2005-12-13 Conexant Systems, Inc. Slotted synchronous frequency division multiplexing for multi-drop networks
US20020034196A1 (en) * 2000-08-01 2002-03-21 Tzannes Marcos C. Systems and methods for transporting a network timing reference in an ADSL system
KR100713470B1 (en) * 2000-08-03 2007-04-30 삼성전자주식회사 Apparatus for pn acqusition in multi-carrier communication system
CA2361247C (en) * 2000-11-06 2008-10-07 Ntt Docomo, Inc. Transmitter, transmitting method, receiver, and receiving method for mc-cdma communication system
US6999508B1 (en) * 2000-11-07 2006-02-14 Texas Instruments Incorporated Multi-channel frequency domain equalizer for radio frequency interference cancellation
US6438367B1 (en) * 2000-11-09 2002-08-20 Magis Networks, Inc. Transmission security for wireless communications
US7023881B1 (en) * 2000-12-22 2006-04-04 Applied Micro Circuits Corporation System and method for selectively scrambling multidimensional digital frame structure communications
KR100363174B1 (en) * 2001-02-15 2002-12-05 삼성전자 주식회사 Apparatus for controlling image display and method thereof
DE10115221A1 (en) * 2001-03-28 2002-10-10 Bosch Gmbh Robert Method for frame and frequency synchronization of an OFDM signal and method for transmitting an OFDM signal
US20040240535A1 (en) * 2001-05-10 2004-12-02 Amit Verma Fast exchange during intialization in multicarrier communication systems
US8291457B2 (en) 2001-05-24 2012-10-16 Vixs Systems, Inc. Channel selection in a multimedia system
US20090031419A1 (en) 2001-05-24 2009-01-29 Indra Laksono Multimedia system and server and methods for use therewith
US7548875B2 (en) 2001-06-27 2009-06-16 John Mikkelsen Media delivery platform
US7010028B2 (en) * 2001-07-16 2006-03-07 Broadcom Corporation System and method for rapid generation of low par Q-mode signals
US7161987B2 (en) 2001-09-26 2007-01-09 Conexant, Inc. Single-carrier to multi-carrier wireless architecture
US7170880B2 (en) * 2001-10-16 2007-01-30 Conexant, Inc. Sample rate change between single-carrier and multi-carrier waveforms
CN1613194A (en) * 2002-01-07 2005-05-04 皇家飞利浦电子股份有限公司 Mobile terminal for cell searching via an iterative correlation system
JP3860762B2 (en) * 2002-02-14 2006-12-20 株式会社エヌ・ティ・ティ・ドコモ Mobile communication system, channel synchronization establishment method, and mobile station
US7376157B1 (en) * 2002-03-08 2008-05-20 Centillium Communications, Inc. Synchronizing ADSL Annex C transceivers to TTR
US7050521B1 (en) * 2002-05-13 2006-05-23 Analog Devices, Inc. Frequency assisted digital timing recovery
KR100476896B1 (en) * 2002-05-22 2005-03-17 삼성전자주식회사 Telecommunication system witch uses digital-subscriber lines coexisting with tcm-isdn line and method of frame synchronization
US7062282B2 (en) * 2002-07-19 2006-06-13 Mediatek, Inc. Method and apparatus for frequency synchronization in a digital transmission system
KR101128043B1 (en) * 2002-08-14 2012-03-29 오아시스 실리콘 시스템즈 Communication system for sending and receiving data onto and from a network at a network frame rate using a phase locked loop, sample rate conversion, or synchronizing clocks generated from the network frame rate
TW583855B (en) * 2002-08-22 2004-04-11 Mediatek Inc Wireless communication device for transmitting RF signals
JP4318510B2 (en) * 2002-08-28 2009-08-26 パナソニック株式会社 Communication apparatus and communication method
JP3480466B2 (en) * 2002-09-17 2003-12-22 富士通株式会社 Digital subscriber line transmission method and xDSL device
US7551640B1 (en) * 2002-09-20 2009-06-23 Cisco Technology, Inc. Method and apparatus for errorless frame timing adjustment
KR100920737B1 (en) * 2002-10-19 2009-10-07 삼성전자주식회사 Multi-carrier transmission system capable of improving reception efficiency of multi-carrier receiver and a method thereof
JP3973543B2 (en) * 2002-11-20 2007-09-12 三洋電機株式会社 Reception method and apparatus
GB2422278B (en) * 2002-12-03 2007-04-04 Synad Technologies Ltd Method and device for synchronisation in OFDM
JP3480469B2 (en) * 2003-02-14 2003-12-22 富士通株式会社 Digital subscriber line transmission method
JP3573152B2 (en) * 2003-02-14 2004-10-06 富士通株式会社 Digital subscriber line transmission method and xDSL device
US8230094B1 (en) * 2003-04-29 2012-07-24 Aol Inc. Media file format, system, and method
US7385617B2 (en) * 2003-05-07 2008-06-10 Illinois Institute Of Technology Methods for multi-user broadband wireless channel estimation
US7342980B2 (en) * 2003-12-30 2008-03-11 Intel Corporation Estimating carrier phase in communication systems
US7817730B2 (en) * 2004-06-09 2010-10-19 Marvell World Trade Ltd. Training sequence for symbol boundary detection in a multicarrier data transmission system
US7643582B2 (en) 2004-06-09 2010-01-05 Marvell World Trade Ltd. Method and system for determining symbol boundary timing in a multicarrier data transmission system
US7561626B2 (en) * 2004-06-09 2009-07-14 Marvell World Trade Ltd. Method and system for channel estimation in a data transmission system
US7697619B2 (en) * 2004-06-09 2010-04-13 Marvell World Trade Ltd. Training sequence for channel estimation in a data transmission system
US7492736B2 (en) * 2004-10-29 2009-02-17 Texas Instruments Incorporated System and method for access and management of beacon periods in distributed wireless networks
US7561627B2 (en) * 2005-01-06 2009-07-14 Marvell World Trade Ltd. Method and system for channel equalization and crosstalk estimation in a multicarrier data transmission system
US7564775B2 (en) 2005-04-29 2009-07-21 Qualcomm, Incorporated Timing control in orthogonal frequency division multiplex systems based on effective signal-to-noise ratio
WO2007022362A2 (en) * 2005-08-16 2007-02-22 Wionics Research Frame synchronization
KR101124027B1 (en) * 2006-08-18 2012-03-23 후지쯔 가부시끼가이샤 Radio resource management in multihop relay networks
CN101179372B (en) * 2006-11-07 2010-09-15 海能达通信股份有限公司 Link frame synchronization system and method of digital communication system
US7787523B2 (en) * 2006-12-01 2010-08-31 Broadcom Corporation Method and system for delay matching in a rake receiver for WCDMA closed loop modes
EP3070863A1 (en) * 2007-02-23 2016-09-21 INOVA Semiconductors GmbH Method of and device for transmitting a serial data frame
KR100899139B1 (en) 2007-07-19 2009-05-27 인하대학교 산학협력단 Receiver for Frame Synchronization in Wireless Communication Systems and Method thereof
GB2454262B (en) * 2007-11-05 2011-02-09 British Broadcasting Corp Signal discovery
JP2009118388A (en) * 2007-11-09 2009-05-28 Nec Electronics Corp Receiver
US8583067B2 (en) * 2008-09-24 2013-11-12 Honeywell International Inc. Apparatus and method for improved wireless communication reliability and performance in process control systems
US20100183060A1 (en) * 2009-01-19 2010-07-22 Electronics And Telecommunications Research Institute Method and apparatus of frequency offset-free frame synchronization for high order qam signals in modem apparatus
AU2010284120B2 (en) * 2009-08-20 2016-06-02 Alfred E. Mann Foundation For Scientific Research Optimal narrowband interference removal for signals separated in time
CN101742013B (en) * 2009-12-22 2012-12-05 中兴通讯股份有限公司 System and method for simulating user line access multiplexing
EP2341654B1 (en) * 2009-12-30 2016-09-14 Lantiq Deutschland GmbH Bit allocation in a multicarrier system
WO2013003804A2 (en) 2011-06-30 2013-01-03 Lutron Electronics Co., Inc. Method for programming a load control device using a smart phone
US9386666B2 (en) 2011-06-30 2016-07-05 Lutron Electronics Co., Inc. Method of optically transmitting digital information from a smart phone to a control device
WO2013012547A1 (en) 2011-06-30 2013-01-24 Lutron Electronics Co., Inc. Load control device having internet connectivity, and method of programming the same using a smart phone
US20130222122A1 (en) 2011-08-29 2013-08-29 Lutron Electronics Co., Inc. Two-Part Load Control System Mountable To A Single Electrical Wallbox
US20140161000A1 (en) * 2012-12-10 2014-06-12 Futurewei Technologies, Inc. Timing offset correction in a tdd vectored system
US9413171B2 (en) 2012-12-21 2016-08-09 Lutron Electronics Co., Inc. Network access coordination of load control devices
US10244086B2 (en) 2012-12-21 2019-03-26 Lutron Electronics Co., Inc. Multiple network access load control devices
US10019047B2 (en) 2012-12-21 2018-07-10 Lutron Electronics Co., Inc. Operational coordination of load control devices for control of electrical loads
US10135629B2 (en) 2013-03-15 2018-11-20 Lutron Electronics Co., Inc. Load control device user interface and database management using near field communication (NFC)
WO2017005295A1 (en) * 2015-07-06 2017-01-12 Telefonaktiebolaget Lm Ericsson (Publ) Resource allocation for data transmission in wireless systems
CN109219946B (en) * 2016-03-31 2021-09-14 弗劳恩霍夫应用研究促进协会 Method, receiver and computer-readable storage medium for receiving data packets
US10447338B2 (en) * 2016-09-23 2019-10-15 Microsoft Technology Licensing, Llc Orthogonal spreading sequence creation using radio frequency parameters
US10020838B2 (en) 2016-09-23 2018-07-10 Microsoft Technology Licensing, Llc Sequence generation for spread spectrum from signal sampling
US11764940B2 (en) 2019-01-10 2023-09-19 Duality Technologies, Inc. Secure search of secret data in a semi-trusted environment using homomorphic encryption

Citations (16)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4450558A (en) 1981-01-12 1984-05-22 General Datacomm Industries, Inc. Method and apparatus for establishing frame synchronization
US4644523A (en) 1984-03-23 1987-02-17 Sangamo Weston, Inc. System for improving signal-to-noise ratio in a direct sequence spread spectrum signal receiver
US5170413A (en) 1990-12-24 1992-12-08 Motorola, Inc. Control strategy for reuse system assignments and handoff
EP0529421A2 (en) 1991-08-29 1993-03-03 Daimler-Benz Aktiengesellschaft Method and apparatus for measuring carrier frequency offset in a multichannel communications system
WO1993009617A1 (en) 1991-11-01 1993-05-13 Motorola, Inc. Multichannel tdm communication system slot phase correction
US5305349A (en) 1993-04-29 1994-04-19 Ericsson Ge Mobile Communications Inc. Quantized coherent rake receiver
US5345440A (en) 1990-09-14 1994-09-06 National Transcommunications Limited Reception of orthogonal frequency division multiplexed signals
WO1995003656A1 (en) 1993-07-20 1995-02-02 Telia Ab Method and apparatus for synchronization in digital transmission systems of the ofdm type
WO1995005042A1 (en) 1993-08-11 1995-02-16 The University Of British Columbia Method and apparatus for frame synchronization in mobile ofdm data communication
US5400322A (en) 1993-08-20 1995-03-21 Amati Communications Corp. Updating of bit allocations in a multicarrier modulation transmission system
US5440561A (en) 1991-09-30 1995-08-08 Motorola, Inc. Method for establishing frame synchronization within a TDMA communication system
US5479447A (en) 1993-05-03 1995-12-26 The Board Of Trustees Of The Leland Stanford, Junior University Method and apparatus for adaptive, variable bandwidth, high-speed data transmission of a multicarrier signal over digital subscriber lines
US5590160A (en) 1992-12-30 1996-12-31 Nokia Mobile Phones Ltd. Symbol and frame synchronization in both a TDMA system and a CDMA
US5596604A (en) 1993-08-17 1997-01-21 Amati Communications Corporation Multicarrier modulation transmission system with variable delay
US5666378A (en) 1994-03-18 1997-09-09 Glenayre Electronics, Inc. High performance modem using pilot symbols for equalization and frame synchronization
US5673296A (en) 1992-04-02 1997-09-30 Nec Corporation Frame synchronization circuit for digital communication system

Family Cites Families (14)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4203071A (en) * 1978-08-08 1980-05-13 The Charles Stark Draper Laboratory, Inc. Pseudo-random-number-code-detection and tracking system
US4365338A (en) * 1980-06-27 1982-12-21 Harris Corporation Technique for high rate digital transmission over a dynamic dispersive channel
US4888769A (en) * 1985-12-06 1989-12-19 Tiw Systems, Inc. TDMA terminal controller
JPH0666745B2 (en) * 1988-04-06 1994-08-24 日本電気株式会社 Frame synchronization method
FR2658017B1 (en) 1990-02-06 1992-06-05 France Etat METHOD FOR BROADCASTING DIGITAL DATA, ESPECIALLY FOR BROADBAND BROADCASTING TO MOBILES, WITH TIME-FREQUENCY INTERLACING AND ASSISTING THE ACQUISITION OF AUTOMATIC FREQUENCY CONTROL, AND CORRESPONDING RECEIVER.
US5343499A (en) 1990-06-12 1994-08-30 Motorola, Inc. Quadrature amplitude modulation synchronization method
SG44771A1 (en) * 1991-02-28 1997-12-19 Philips Electronics Nv System for broadcasting and receiving digital data receiver and transmitter for use in such system
KR100236809B1 (en) * 1991-09-30 2000-01-15 벤 씨. 카덴헤드 Microfiltration of zeolites
FR2682249B1 (en) * 1991-10-08 1993-12-03 Thomson Csf METHOD FOR THE DIGITAL DEMODULATION OF A COMPOSITE SIGNAL.
US5285474A (en) * 1992-06-12 1994-02-08 The Board Of Trustees Of The Leland Stanford, Junior University Method for equalizing a multicarrier signal in a multicarrier communication system
US5299236A (en) * 1992-11-13 1994-03-29 Toshiba America Information Systems, Inc. System and method for obtaining and maintaining synchronization of a demodulated signal
ES2159540T3 (en) * 1993-02-08 2001-10-16 Koninkl Philips Electronics Nv RECEIVER, WITH MULTIPLEXOR OF ORTOGONAL FREQUENCY DIVISION, WITH COMPENSATION FOR DIFFERENTIAL DELAYS.
US5386590A (en) * 1993-07-28 1995-01-31 Celeritas Technologies, Ltd. Apparatus and method for increasing data transmission rate over wireless communication systems using spectral shaping
US6549512B2 (en) * 1997-06-25 2003-04-15 Texas Instruments Incorporated MDSL DMT architecture

Patent Citations (17)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4450558A (en) 1981-01-12 1984-05-22 General Datacomm Industries, Inc. Method and apparatus for establishing frame synchronization
US4644523A (en) 1984-03-23 1987-02-17 Sangamo Weston, Inc. System for improving signal-to-noise ratio in a direct sequence spread spectrum signal receiver
US5345440A (en) 1990-09-14 1994-09-06 National Transcommunications Limited Reception of orthogonal frequency division multiplexed signals
US5170413A (en) 1990-12-24 1992-12-08 Motorola, Inc. Control strategy for reuse system assignments and handoff
EP0529421A2 (en) 1991-08-29 1993-03-03 Daimler-Benz Aktiengesellschaft Method and apparatus for measuring carrier frequency offset in a multichannel communications system
US5440561A (en) 1991-09-30 1995-08-08 Motorola, Inc. Method for establishing frame synchronization within a TDMA communication system
WO1993009617A1 (en) 1991-11-01 1993-05-13 Motorola, Inc. Multichannel tdm communication system slot phase correction
US5673296A (en) 1992-04-02 1997-09-30 Nec Corporation Frame synchronization circuit for digital communication system
US5590160A (en) 1992-12-30 1996-12-31 Nokia Mobile Phones Ltd. Symbol and frame synchronization in both a TDMA system and a CDMA
US5305349A (en) 1993-04-29 1994-04-19 Ericsson Ge Mobile Communications Inc. Quantized coherent rake receiver
US5479447A (en) 1993-05-03 1995-12-26 The Board Of Trustees Of The Leland Stanford, Junior University Method and apparatus for adaptive, variable bandwidth, high-speed data transmission of a multicarrier signal over digital subscriber lines
WO1995003656A1 (en) 1993-07-20 1995-02-02 Telia Ab Method and apparatus for synchronization in digital transmission systems of the ofdm type
US5444697A (en) 1993-08-11 1995-08-22 The University Of British Columbia Method and apparatus for frame synchronization in mobile OFDM data communication
WO1995005042A1 (en) 1993-08-11 1995-02-16 The University Of British Columbia Method and apparatus for frame synchronization in mobile ofdm data communication
US5596604A (en) 1993-08-17 1997-01-21 Amati Communications Corporation Multicarrier modulation transmission system with variable delay
US5400322A (en) 1993-08-20 1995-03-21 Amati Communications Corp. Updating of bit allocations in a multicarrier modulation transmission system
US5666378A (en) 1994-03-18 1997-09-09 Glenayre Electronics, Inc. High performance modem using pilot symbols for equalization and frame synchronization

Non-Patent Citations (6)

* Cited by examiner, † Cited by third party
Title
Chow et al. "Equalizer Training Algorithms for Multicarrier Modulation", (1993), pp. 761-765.
Chow et al., "A Discrete Multitone Transceiver System for HDSL Applications", IEEE Journ. on Sel. areas in Comm., vol. 9, No. 6, Aug. 1991, pp. 895-908.
Chow et al., "DMT Initialization: Parameters Needed for Specification in a Standard", Mar. 1993. pp. 1-8. T1E1.4, ANSI Standards Committee (Submission T1E1.4/93-0222).
John A.C. Bingham, "Multicarrier Modulation for Data Transmission: An Idea Whose Time Has come", May 1990, pp. 5-14.
Kirkman et al., "Detection of Characteristic Phases Using FFT's", International Conference on Acoustics, Speech and Signal Processing IEEE, 3/92.
Leonard J. Cimini, Jr., "Analysis and Simulation of a Digital Mobile Channel Using Orthogonal Frequency Division Multiplexing", Jul. 7, 1985, IEEE Transactions on Communications, vol. COM-33, No. 7.

Cited By (24)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US6993084B1 (en) * 1998-04-14 2006-01-31 Fraunhofer-Gesellschaft Zur Foerderung Der Angewandten Forschung E.V. Coarse frequency synchronisation in multicarrier systems
US6785349B1 (en) * 1999-05-28 2004-08-31 3Com Corporation Correlation based method of determining frame boundaries of data frames that are periodically extended
US6813325B1 (en) * 1999-12-22 2004-11-02 Globespanvirata, Inc System and method to reduce transmit wander in a digital subscriber line
US20020048333A1 (en) * 2000-05-25 2002-04-25 Nadeem Ahmed Joint detection in OFDM systems
US6885696B2 (en) * 2000-07-28 2005-04-26 Nortel Networks Limited Notifying bit allocation changes in a multicarrier modulation communications system
US20020080824A1 (en) * 2000-07-28 2002-06-27 Wingrove Michael J. Notifying bit allocation changes in a multicarrier modulation communications system
US20020110208A1 (en) * 2001-02-09 2002-08-15 Alps Electric Co., Ltd. Frame synchronizing signal detecting method for reducing occurrence of error synchronization before link of frame synchronizing signal is established
US7027541B2 (en) * 2001-02-09 2006-04-11 Alps Electric Co., Ltd. Frame synchronizing signal detecting method for reducing occurrence of error synchronization before link of frame synchronizing signal is established
US20020159553A1 (en) * 2001-04-17 2002-10-31 Comspace Corporation Method and apparatus for improving data frame synchronization in a low SNR environment
US7012981B2 (en) * 2001-04-17 2006-03-14 Cyntrust Communications, Inc. Method and apparatus for improving data frame synchronization in a low SNR environment
US20110317671A1 (en) * 2001-10-18 2011-12-29 Qualcomm Incorporated Multiple-access hybrid ofdm-cdma system
US8948705B2 (en) * 2001-10-18 2015-02-03 Qualcomm Incorporated Multiple-access hybrid OFDM-CDMA system
US20030103529A1 (en) * 2001-11-30 2003-06-05 Niklas Linkewitsch Deframer
US7215685B2 (en) * 2001-11-30 2007-05-08 Intel Corporation Frame aligning deframer
US6925112B1 (en) * 2002-07-26 2005-08-02 Jabil Circuit, Inc. Discrete multitone modem initialization system and method
WO2004093366A2 (en) * 2003-04-09 2004-10-28 Jabil Circuit, Inc. Method for initialization of per tone frequency domain equalizer (feq) through noise reduction for multi-tone based modems
US7406125B2 (en) 2003-04-09 2008-07-29 Jabil Circuit, Inc. Method for initialization of per tone frequency domain equalizer (FEQ) through noise reduction for multi-tone based modems
WO2004093366A3 (en) * 2003-04-09 2005-02-24 Jabil Circuit Inc Method for initialization of per tone frequency domain equalizer (feq) through noise reduction for multi-tone based modems
US20040264587A1 (en) * 2003-04-09 2004-12-30 Jabil Circuit, Inc. Method for initialization of per tone frequency domain equalizer (FEQ) through noise reduction for multi-tone based modems
US20080056343A1 (en) * 2006-08-30 2008-03-06 Ravikiran Rajagopal Frame synchronization
US20100287402A1 (en) * 2009-05-11 2010-11-11 Electronics And Telecommunications Research Institute Timestamping apparatus and method
US9100261B2 (en) 2013-06-24 2015-08-04 Freescale Semiconductor, Inc. Frequency-domain amplitude normalization for symbol correlation in multi-carrier systems
US9106499B2 (en) 2013-06-24 2015-08-11 Freescale Semiconductor, Inc. Frequency-domain frame synchronization in multi-carrier systems
US9282525B2 (en) 2013-06-24 2016-03-08 Freescale Semiconductor, Inc. Frequency-domain symbol and frame synchronization in multi-carrier systems

Also Published As

Publication number Publication date
CA2347268A1 (en) 1996-02-01
EP0771496B1 (en) 1999-12-08
CN1163036C (en) 2004-08-18
EP0771496A1 (en) 1997-05-07
FI970161A0 (en) 1997-01-15
CN1095260C (en) 2002-11-27
JP3816485B2 (en) 2006-08-30
CA2194972C (en) 2001-09-18
CN1233137C (en) 2005-12-21
KR100417815B1 (en) 2004-05-31
AU696798B2 (en) 1998-09-17
AU3006695A (en) 1996-02-16
KR970705263A (en) 1997-09-06
US20040199554A1 (en) 2004-10-07
CA2347252C (en) 2003-01-14
JPH10502511A (en) 1998-03-03
US20020094049A1 (en) 2002-07-18
DK0771496T3 (en) 2000-05-29
CA2347252A1 (en) 1996-02-01
JP3303055B2 (en) 2002-07-15
FI115685B (en) 2005-06-15
US5901180A (en) 1999-05-04
US6912261B2 (en) 2005-06-28
WO1996002991A1 (en) 1996-02-01
CN1157070A (en) 1997-08-13
DE29521458U1 (en) 1997-05-22
JP3768434B2 (en) 2006-04-19
DE69513834T2 (en) 2000-07-06
FI970161A (en) 1997-03-14
CA2194972A1 (en) 1996-02-01
ES2144132T3 (en) 2000-06-01
JP2004147345A (en) 2004-05-20
CN1352504A (en) 2002-06-05
JP2002208919A (en) 2002-07-26
CN1404271A (en) 2003-03-19
US5627863A (en) 1997-05-06
DE69513834D1 (en) 2000-01-13

Similar Documents

Publication Publication Date Title
US6359933B1 (en) Frame synchronization in muticarrier transmission systems
JP4130995B2 (en) Improvements in or related to multi-carrier transmission systems
US6538986B2 (en) Data transmission system and method using nQAM constellation with a control channel superimposed on a user data channel
US6865232B1 (en) Multi-carrier transmission systems
US6181714B1 (en) Multi-carrier transmission systems
JP4130994B2 (en) Improvements in or related to multi-carrier transmission systems
JP4447056B2 (en) Improvements in or related to multi-carrier transmission systems
JP4130996B2 (en) Improvements in or related to multi-carrier transmission systems
US6320903B1 (en) Multi-carrier transmission systems
JP4016126B2 (en) Improvement of multi-carrier transmission system
US6366554B1 (en) Multi-carrier transmission systems
EP0923822A2 (en) Improvements in, or relating to, multi-carrier transmission systems

Legal Events

Date Code Title Description
STCF Information on status: patent grant

Free format text: PATENTED CASE

FPAY Fee payment

Year of fee payment: 4

FPAY Fee payment

Year of fee payment: 8

FPAY Fee payment

Year of fee payment: 12